DebugInfo: Make gmlt debug info more gmlt-like by removing variables.

For some impending improvements to debug info, LLVM will start assuming
that when the CU specifies llvm::DIBuilder::LineTablesOnly, the IR for
functions described by that CU will not include variables, types, etc.

(might be worth having some test coverage for GMLT + non-GMLT CUs,
especially with non-GMLT functions inlined into GMLT CU functions)