Add a new attribute on records, __attribute__((adl_invisible)), and define

the x86-64 __va_list_tag with this attribute.  The attribute causes the
affected type to behave like a fundamental type when considered by ADL.

(x86-64 is the only target we currently provide with a struct-based
__builtin_va_list)

Fixes PR6762.

llvm-svn: 104941