Summary:
This patch adds support for handling of variadic functions for AIX.
This includes ensuring that use and consume correct type of
va_list (char *va_list) for AIX.
Authored by: ZarkoCA
Reviewers: cebowleratibm, sfertile, jasonliu
Reviewed by: jasonliu
Differential Revision: https://reviews.llvm.org/D76130
