Add MMA builtin decoding. These builtins use the new PowerPC-specific types vector_pair and vector_quad.
So to avoid pervasive changes, we use custom type descriptors and custom decoding of these builtins.
We also use custom code generation to expand builtin calls with pointers to simpler intrinsic calls with non-pointer types only.
Depends on D81508
I think it might be good to add a small comment here/in ASTContext.cpp for this function (in how we use it to fall back to default type handling).