Suppress -Wunused-value within macros from system headers.

Along the way, move a helper function from SemaChecking.cpp to a more
accessible home in SourceManager.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147692 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed