Revert clang change that breaks CTF on aarch64

Revert commit e655e74a318e from llvm git (by Peter Collingbourne):

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

As reported by Jessica in https://reviews.llvm.org/D104830#3129527, this
upstream change is implemented in such a way that it breaks DTrace's
CTF. Since a proper fix has not yet been forthcoming, and we are
unaffected by the (CFI-related) problem upstream was trying to address,
revert the change for now.

Requested by: jrtc27
MFC after: 3 days

(cherry picked from commit da2012af42fb704365cfaff7ae68fc7de59981da)