[Clang] Warn on deprecated specializations used in system headers. (#70353)

When the top of the instantiation stack is in user code.

The goal of this PR is to allow deprecation of some char_traits
specializations in libc++ as done in https://reviews.llvm.org/D157058
which was later reverted by
https://github.com/llvm/llvm-project/pull/66153#issuecomment-1719578384
as Clang never emitted the libc++ warnings.

Because Clang likes to eagerly instantiate, we can look for the location
of the top of the instantiation stack, and emit a warning if that
location is in user code.

The warning emission is forced by temporarily instructing the diag
engine not to silence warning in system headers.