[PowerPC] Add MMA builtin decoding and definitions

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 for these builtins.
We also use custom code generation to expand builtin calls with pointers to simpler intrinsic calls with non-pointer types.

Differential Revision: https://reviews.llvm.org/D81748