[clang] [Headers][X86] Allow AVX _mm256_set* intrinsics to be used in constexpr (PR #152173)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-libcxx` running on `linaro-flang-aarch64-libcxx` while building `clang` at step 11 "test-build-flang-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/89/builds/21296

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

```
Step 11 (test-build-flang-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
0.255 [1/3/1] Linking CXX executable unittests/Common/FlangCommonTests
5.437 [1/2/2] Linking CXX executable unittests/Optimizer/FlangOptimizerTests
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1205.506055

```

</details>

https://github.com/llvm/llvm-project/pull/152173