Sema: relax va_start checking further for Windows AArch64

When building in C mode, the VC runtime assumes that it can use pointer
aliasing through `char *` for the parameter to `__va_start`.  Relax the
checks further.  In theory we could keep the tests strict for non-system
header code, but this takes the less strict approach as the additional
check doesn't particularly end up being too much more helpful for
correctness.  The C++ type system is a bit stricter and requires the
explicit cast which we continue to verify.