[PowerPC] Implement Vector Splat Immediate Builtins in Clang

[PowerPC] Implement Vector Splat Immediate Builtins in Clang

Implements builtins for the following prototypes:
  vector signed int vec_splati (const signed int);
  vector float vec_splati (const float);
  vector double vec_splatid (const float);
  vector signed int vec_splati_ins (vector signed int, const unsigned int,
                                    const signed int);
  vector unsigned int vec_splati_ins (vector unsigned int, const unsigned int,
                                      const unsigned int);
  vector float vec_splati_ins (vector float, const unsigned int, const float);

Differential Revision: https://reviews.llvm.org/D82520