PR13189: va_list broken with precompiled headers

For some targets a structure named __va_list_tag is built to help define
the __builtin_va_list type.  However, __va_list_tag was not being treated as a
predefined type thus causing problems when serializing the AST.  This commit
fixes that oversight by adding the necessary support to treat __va_list_tag
as a predefined type.

llvm-svn: 159508