Prospective build fix for unsafe usage of getCustomDiagID()

This now requires a compile-time constant string so let's build proper
diagnostic IDs and pass through the inputs as arguments.

Tracks clang changes in r200132.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@200139 91177308-0d34-0410-b5e6-96231b3b80d8