[clang] [llvm] Ast importer visitors (PR #138838)

[Balázs Kéri via llvm-commits]

balazske wrote:

If `FromTypeRequirement.isSubstitutionFailure()` can be true at compile error it can be still possible to call `testImport` or `getTuDecl` with the code.

https://github.com/llvm/llvm-project/pull/138838