[clang] [clang-tools-extra] [C2y] Implement WG14 N3409 (PR #130299)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `clang-tools-extra,clang` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/21425

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

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests/134/174' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/b/1/llvm-x86_64-debian-dylib/build/unittests/Support/./SupportTests-LLVM-Unit-3324496-134-174.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=174 GTEST_SHARD_INDEX=134 /b/1/llvm-x86_64-debian-dylib/build/unittests/Support/./SupportTests
--

Script:
--
/b/1/llvm-x86_64-debian-dylib/build/unittests/Support/./SupportTests --gtest_filter=Caching.Normal
--
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/unittests/Support/Caching.cpp:55: Failure
Value of: AddStream
  Actual: false
Expected: true


/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/unittests/Support/Caching.cpp:55
Value of: AddStream
  Actual: false
Expected: true



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


```

</details>

https://github.com/llvm/llvm-project/pull/130299