Fix another invalid getCustomDiagID() use to unbreak the build

It was calling the utility wrapper that now requires a constant string
following clang r200132. The StringRef version on DiagnosticIDs appears to have
been what was intended so change to that.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@200142 91177308-0d34-0410-b5e6-96231b3b80d8