[clang] [clang] add test verifying DR242 behavior (PR #201441)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Thank you for working on this!

The PR description needs more details:
1. CWG242 is not a particularly small issue, so you should have a sentence or two that summarizes it.
2. The resolution is 10 years old at this point, so you definitely should check what the current draft says and report it here.

https://github.com/llvm/llvm-project/pull/201441