Refactor gold-plugin codegen to prepare for ThinLTO threads (NFC)

This is the NFC part remaining from D15390, which refactors the
current codegen() into a CodeGen class with various modular methods and
other helper functions that will be used by the follow-on ThinLTO piece.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262721 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed