[clang-tools-extra] [flang] [clang] [llvm] [compiler-rt] [clang] Add test for CWG1807 (PR #77637)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@mysterymath It's a bit of a shame to admit that I missed the first half of RUN lines in the previous follow-up. Should be fixed now by a7588bb9bab43420f1c2642c80489f74af88f855

https://github.com/llvm/llvm-project/pull/77637