Teach Clang to recognize and ignore GCC's -fdiagnostics-show-location
flag. We should probably wire at least some variants of this up to our
actual diagnostics engine, but I'm leaving that for someone else. This
fixes the builds of packages which hard code something here, at least
including libstdc++ itself.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193318 91177308-0d34-0410-b5e6-96231b3b80d8