AST: Move __va_list tag back to std conditionally on AArch64.

In post-commit feedback on D104830 Jessica Clarke pointed out that
unconditionally adding __va_list to the std namespace caused namespace
debug info to be emitted in C, which is not only inappropriate but
turned out to confuse the dtrace tool. Therefore, move __va_list back
to std only in C++ so that the correct debug info is generated. We
also considered moving __va_list to the top level unconditionally
but this would contradict the specification and be visible to AST
matchers and such, so make it conditional on the language mode.

To avoid breaking name mangling for __va_list, teach the Itanium
name mangler to always mangle it as if it were in the std namespace
when targeting ARM architectures. This logic is not needed for the
Microsoft name mangler because Microsoft platforms define va_list as
a typedef of char *.

Depends on D116773

Differential Revision: https://reviews.llvm.org/D116774