Changes

Changes from Git (git https://github.com/llvm/llvm-project.git)

Summary

  1. Provide anchor for compiler extensions (details)
Commit 38778e1087b2825e91b07ce4570c70815b49dcdc by sguelton
Provide anchor for compiler extensions

This patch is cherry-picked from 04b0a4e22e3b4549f9d241f8a9f37eebecb62a31, and
amended to prevent an undefined reference to `llvm::EnableABIBreakingChecks'
The file was modifiedllvm/lib/Extensions/Extensions.cpp
The file was modifiedllvm/lib/Extensions/LLVMBuild.txt