[clang] fix deduction of member pointers with dependent named classes (#133113)
This fixes a regression when interpreting a nested name specifier for
deduction purposes in member pointers.
This introduces a helper for fully translating a nested name specifier
into a type.
This regression was introduced here:
https://github.com/llvm/llvm-project/pull/130537
and was reported here:
https://github.com/llvm/llvm-project/pull/132401#issuecomment-2751489581
No release notes, since this regression was never released.