[llvm] [ADT] Add `DefaultUnreachable("msg")` to StringSwitch (PR #161976)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `bolt-aarch64-ubuntu-clang` running on `bolt-worker-aarch64` while building `llvm` at step 5 "build-clang-bolt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/128/builds/7527

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-clang-bolt) failure: build (failure)
...
512.041 [8/2/3330] Linking CXX executable bin/llvm-bolt
534.980 [8/1/3331] Building CXX object tools/llvm-readobj/CMakeFiles/llvm-readobj.dir/ELFDumper.cpp.o
535.104 [7/1/3332] Linking CXX executable bin/llvm-readobj
535.112 [2/5/3333] Building CXX object tools/clang/tools/driver/CMakeFiles/clang.dir/clang-driver.cpp.o
537.942 [2/4/3334] Building CXX object tools/clang/tools/driver/CMakeFiles/clang.dir/cc1gen_reproducer_main.cpp.o
539.611 [2/3/3335] Building CXX object tools/clang/tools/driver/CMakeFiles/clang.dir/cc1as_main.cpp.o
541.621 [2/2/3336] Building CXX object tools/clang/tools/driver/CMakeFiles/clang.dir/driver.cpp.o
545.146 [2/1/3337] Building CXX object tools/clang/tools/driver/CMakeFiles/clang.dir/cc1_main.cpp.o
545.982 [1/1/3338] Linking CXX executable bin/clang-22
930.158 [0/1/3339] Creating executable symlink bin/clang
FAILED: bin/clang 
/usr/bin/cmake -E cmake_symlink_executable bin/clang-22 bin/clang && cd /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver && /usr/bin/cmake -E create_symlink clang-22 /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/clang++ && cd /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver && /usr/bin/cmake -E create_symlink clang-22 /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/clang-cl && cd /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver && /usr/bin/cmake -E create_symlink clang-22 /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/clang-cpp && cd /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver && /usr/bin/python3 /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/llvm-project/clang/tools/driver/../../utils/perf-training/perf-helper.py bolt-optimize --method INSTRUMENT --input /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/clang-22 --instrumented-output /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/clang-bolt.inst --fdata /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver/../../utils/perf-training/prof.fdata --perf-training-binary-dir /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver/../../utils/perf-training --readelf /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/llvm-readobj --bolt /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/llvm-bolt --lit /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/llvm-lit --merge-fdata /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/merge-fdata
Running: /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/llvm-bolt /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/bin/clang-22 -o /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/clang-bolt.inst -instrument --instrumentation-file-append-pid --instrumentation-file=/home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver/../../utils/perf-training/prof.fdata
BOLT-INFO: shared object or position-independent executable detected
BOLT-INFO: Target architecture: aarch64
BOLT-INFO: BOLT version: <unknown>
BOLT-INFO: first alloc address is 0x0
BOLT-INFO: creating new program header table at address 0x8800000, offset 0x8800000
BOLT-INFO: enabling relocation mode
BOLT-INFO: forcing -jump-tables=move for instrumentation
BOLT-WARNING: 1 collisions detected while hashing binary objects. Use -v=1 to see the list.
BOLT-INFO: number of removed linker-inserted veneers: 0
BOLT-INFO: 0 out of 153649 functions in the binary (0.0%) have non-empty execution profile
BOLT-INSTRUMENTER: Number of indirect call site descriptors: 60398
BOLT-INSTRUMENTER: Number of indirect call target descriptors: 151473
BOLT-INSTRUMENTER: Number of function descriptors: 151465
BOLT-INSTRUMENTER: Number of branch counters: 1991924
BOLT-INSTRUMENTER: Number of ST leaf node counters: 1015407
BOLT-INSTRUMENTER: Number of direct call counters: 0
BOLT-INSTRUMENTER: Total number of counters: 3007331
BOLT-INSTRUMENTER: Total size of counters: 24058648 bytes (static alloc memory)
BOLT-INSTRUMENTER: Total size of string table emitted: 15800082 bytes in file
BOLT-INSTRUMENTER: Total size of descriptors: 222201716 bytes in file
BOLT-INSTRUMENTER: Profile will be saved to file /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver/../../utils/perf-training/prof.fdata
BOLT-INFO: removed 22147 empty blocks
BOLT-INFO: merged 4 duplicate CFG edges
BOLT-INFO: Starting stub-insertion pass
BOLT-INFO: Inserted 1463 stubs in the hot area and 0 stubs in the cold area. Shared 164016 times, iterated 4 times.
BOLT-INFO: padding code to 0x12e00000 to accommodate hot text
BOLT-INFO: output linked against instrumentation runtime library, lib entry point is 0x159d88c0
BOLT-INFO: clear procedure is 0x159d7540
BOLT-INFO: patched build-id (flipped last bit)
BOLT-INFO: setting __bolt_runtime_start to 0x159d88c0
BOLT-INFO: setting __bolt_runtime_fini to 0x159d8954
BOLT-INFO: setting __hot_start to 0x8a00000
BOLT-INFO: setting __hot_end to 0x12cf82e8
Running: /usr/bin/python3 /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/./bin/llvm-lit -v /home/buildbot/workspace/bolt-aarch64-ubuntu-clang/build/tools/clang/tools/driver/../../utils/perf-training/bolt-fdata
-- Testing: 2 tests, 2 workers --
PASS: Clang Perf Training :: cxx/hello_world.cpp (1 of 2)

```

</details>

https://github.com/llvm/llvm-project/pull/161976