AST: Create __va_list in the std namespace even in C.

This ensures that the mangled type names match between C and C++,
which is significant when using -fsanitize=cfi-icall. Ideally we
wouldn't have created this namespace at all, but it's now part of
the ABI (e.g. in mangled names), so we can't change it.

Differential Revision: https://reviews.llvm.org/D104830