[clang] [libcxx] [llvm] Reapply "[Clang] Implement resolution for CWG1835 (#92957)" (PR #98547)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> WDYAT?

We generally try to avoid issuing more than one error diagnostic for the same problem.

Couldn't we instead issue a note `note: maybe missing 'template' keyword prior to dependent template name 'A'`
attached to the first error instead?

https://github.com/llvm/llvm-project/pull/98547