[clang] [clang] fix unresolved dependent template specialization mangling (PR #135111)

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

https://godbolt.org/z/sY44dG6Ya is the reproducer. It is not super small but still should give an idea as the stack is quite similar to stuff touched by this PR.

https://github.com/llvm/llvm-project/pull/135111