OptDiag: Decouple backend diagnostics from debug info metadata

This creates and uses a DiagnosticLocation type rather than using
DebugLoc for this purpose in the backend diagnostics. This is NFC for
now, but will allow us to create locations for diagnostics without
having to create new metadata nodes when we don't have a DILocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295519 91177308-0d34-0410-b5e6-96231b3b80d8

Author: bogner