[codeview] Emit qualified display names if -gline-tables-only is on

When -gmlt is on, we don't emit namespace or class scope information,
and the CodeView emission code in LLVM can't compute the fully qualified
name. If we know LLVM won't be able to get the name right, go ahead and
emit the qualified name in the frontend.

We could change our -gmlt emission strategy to include those scopes when
emitting codeview, but that would increase memory usage and slow down
LTO and add more complexity to debug info emission.

The same problem exists when you debug a -gmlt binary with GDB, so we
should consider removing '&& EmitCodeView' from the condition here at
some point in the future after evaluating the impact on object file
size.

llvm-svn: 274246