[COFF, ARM64] Add __getReg intrinsic
Reviewers: rnk, mstorsjo, compnerd, TomTan, haripul, javed.absar, efriedma
Reviewed By: efriedma
Subscribers: peter.smith, efriedma, kristof.beyls, chrib, cfe-commits
Differential Revision: https://reviews.llvm.org/D52838