[clang] Fix reading long doubles with va_arg on x86_64 mingw

On x86_64 mingw, long doubles are always passed indirectly as
arguments (see an existing case in WinX86_64ABIInfo::classify);
generalize the existing code for reading varargs - any non-aggregate
type that is larger than 64 bits (which would be both long double
in mingw, and __int128) are passed indirectly too.

This makes reading varargs consistent with how they're passed,
fixing interop with both gcc and clang callers, for long double
and __int128.

Differential Revision: https://reviews.llvm.org/D103452