CommentSema: Factor code better. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194354 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid double StringMap lookups. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194355 91177308-0d34-0410-b5e6-96231b3b80d8