[clang] Apply MS ABI details on __builtin_ms_va_list on non-windows platforms on x86_64

This fixes inconsistencies in the ms_abi.c testcase.

Also add a couple cases of missing double pointers in the windows part
of the testcase; the outcome of building that testcase on windows hasn't
changed, but the previous form of the test was imprecise (checking
for "%[[STRUCT_FOO]]*" when clang actually generates "%[[STRUCT_FOO]]**"),
which still used to match.

Ideally this would share code with the native Windows case, but
X86_64ABIInfo and WinX86_64ABIInfo aren't superclasses/subclasses of
each other so it's impractical, and the code to share currently only
consists of a couple lines.

Differential Revision: https://reviews.llvm.org/D103837