[clang][Headers] Do not define varargs macros for __need___va_list

The glibc uses the define to avoid namespace polution on headers
that requires variadic argument, where the inclusion of stdarg.h is
required to obtain the va_list definition.

For such cases only __gnuc_va_list is required.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D137268