[Clang][PowerPC] Add __vector_pair and __vector_quad types

Define the __vector_pair and __vector_quad types that are used to manipulate
the new accumulator registers introduced by MMA on PowerPC. Because these two
types are specific to PowerPC, they are defined in a separate new file so it
will be easier to add other PowerPC specific types if we need to in the future.

Differential Revision: https://reviews.llvm.org/D81508