[NFC]Fix IR/MC depency issue for function descriptor SDAG implementation

Summary: llvm/IR/GlobalValue.h can't be included in MC, that creates a circular dependency between MC and IR libraries. This circular dependency is causing an issue for build system that enforce layering.

Author: Xiangling_L

Reviewers: sfertile, jasonliu, hubert.reinterpretcast, gribozavr

Reviewed By: gribozavr

Subscribers: wuzish, nemanjai, hiraditya, kbarton, MaskRay, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64445

llvm-svn: 365701