[llvm] Remove unused <vector> inclusion (PR #166997)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/25211

<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/12/36' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/CAS/./CASTests-LLVM-Unit-349260-12-36.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=36 GTEST_SHARD_INDEX=12 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/CAS/./CASTests
--

Script:
--
/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/unittests/CAS/./CASTests --gtest_filter=CASProgramTest.MappedFileRegionArenaTest
--
../llvm/llvm/unittests/CAS/ProgramTest.cpp:147: Failure
Value of: PI.ReturnCode == 0
  Actual: false
Expected: true


../llvm/llvm/unittests/CAS/ProgramTest.cpp:147
Value of: PI.ReturnCode == 0
  Actual: false
Expected: true



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


```

</details>

https://github.com/llvm/llvm-project/pull/166997