[llvm] [ADT] Simplify ResolveUnderlyingType (NFC) (PR #164114)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-m68k-linux-cross` running on `suse-gary-m68k-cross` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/27/builds/17710

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM-Unit :: CAS/./CASTests/2/17' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/unittests/CAS/./CASTests-LLVM-Unit-3204612-2-17.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=17 GTEST_SHARD_INDEX=2 /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/unittests/CAS/./CASTests
--

Script:
--
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/unittests/CAS/./CASTests --gtest_filter=CASProgramTest.MappedFileRegionArenaTest
--
Note: Google Test filter = CASProgramTest.MappedFileRegionArenaTest
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from CASProgramTest
/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CAS/ProgramTest.cpp:147: Failure
Value of: PI.ReturnCode == 0
  Actual: false
Expected: true


/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/llvm/llvm/unittests/CAS/ProgramTest.cpp:147
Value of: PI.ReturnCode == 0
  Actual: false
Expected: true



********************


```

</details>

https://github.com/llvm/llvm-project/pull/164114