[libcxx] Add local %T substitution
This patch adds a %T substitution directly into the libc++ test format.
This ensures that the libc++ test format will continue to work when we
remove support for %T in llvm lit.
Reviewers: #reviewers-libcxx, ldionne, philnik777
Pull Request: https://github.com/llvm/llvm-project/pull/160026
