[clang] [clang] Use llvm::SmallVector::pop_back_val (NFC) (PR #136451)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-test-suite` running on `ppc64le-clang-test-suite` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/95/builds/12232

<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 :: Support/./SupportTests/948/1514' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/build/unittests/Support/./SupportTests-LLVM-Unit-904000-948-1514.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=1514 GTEST_SHARD_INDEX=948 /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/build/unittests/Support/./SupportTests
--

Script:
--
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/build/unittests/Support/./SupportTests --gtest_filter=ProgramEnvTest.TestLockFile
--
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/unittests/Support/ProgramTest.cpp:570: Failure
Value of: Error.empty()
  Actual: false
Expected: true


/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-test-suite/clang-ppc64le-test-suite/llvm-project/llvm/unittests/Support/ProgramTest.cpp:570
Value of: Error.empty()
  Actual: false
Expected: true



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


```

</details>

https://github.com/llvm/llvm-project/pull/136451