[llvm] Add a pass to collect dropped var stats for MIR (PR #120501)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/14326

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: MIR/./MIRTests/2/17' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests-LLVM-Unit-2111083-2-17.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=17 GTEST_SHARD_INDEX=2 /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests
--

Note: This is test shard 3 of 17.
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from DroppedVariableStatsMIR
[ RUN      ] DroppedVariableStatsMIR.UnrelatedScopes
 #0 0x00005aa0769a1980 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x118d980)
 #1 0x00005aa07699efaf llvm::sys::RunSignalHandlers() (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x118afaf)
 #2 0x00005aa07699f105 SignalHandler(int) Signals.cpp:0:0
 #3 0x00007c9c7bdd8520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00005aa076181792 llvm::MachineModuleInfo::MachineModuleInfo(llvm::TargetMachine const*) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x96d792)
 #5 0x00005aa0759f36d9 (anonymous namespace)::DroppedVariableStatsMIR_UnrelatedScopes_Test::TestBody() DroppedVariableStatsMIRTest.cpp:0:0
 #6 0x00005aa0769c56c3 testing::Test::Run() (.part.0) gtest-all.cc:0:0
 #7 0x00005aa0769cfc32 testing::TestInfo::Run() (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x11bbc32)
 #8 0x00005aa0769d01d1 testing::TestSuite::Run() (.part.0) gtest-all.cc:0:0
 #9 0x00005aa0769daa5a testing::internal::UnitTestImpl::RunAllTests() (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x11c6a5a)
#10 0x00005aa0769c4a2f testing::UnitTest::Run() (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x11b0a2f)
#11 0x00005aa0759dadaa main (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x1c6daa)
#12 0x00007c9c7bdbfd90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#13 0x00007c9c7bdbfe40 call_init ./csu/../csu/libc-start.c:128:20
#14 0x00007c9c7bdbfe40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#15 0x00005aa0759ef0d5 _start (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests+0x1db0d5)

--
exit: -11
--
shard JSON output does not exist: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/unittests/MIR/./MIRTests-LLVM-Unit-2111083-2-17.json
********************


```

</details>

https://github.com/llvm/llvm-project/pull/120501