Add blurb about DXR.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_34@195313 91177308-0d34-0410-b5e6-96231b3b80d8
Add blurb about Likely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_34@195315 91177308-0d34-0410-b5e6-96231b3b80d8