[clang] [clang] Migrate DR tests to `static_assert` (PR #88611)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

This PR removes a FIXME since it doesn't make sense anymore in that particular context. I created issue #88608 out of it to make sure we don't lose track of it.

https://github.com/llvm/llvm-project/pull/88611