[MS] Fix 'this' type when calling virtual methods with inalloca

If the virtual method comes from a secondary vtable, then the type of
the 'this' parameter should be i8*, and not a pointer to the complete
class. In the MS ABI, the 'this' parameter on entry points to the vptr
containing the virtual method that was called, so we use i8* instead of
the normal type. We had a mismatch where the CGFunctionInfo of the call
didn't match the CGFunctionInfo of the declaration, and this resulted in
some assertions, but now both sides agree the type of 'this' is i8*.

Fixes one issue raised in PR30293

llvm-svn: 280815