DebugInfo: Omit an empty CU when a subprogram was moved into its use

When the only use of a CU is for a subprogram that's only emitted into
the using CU (to avoid cross-CU references in DWO files), avoid creating
that CU at all.

Reapply of r304111 after adding a triple to the test and moving it
to the X86 directory.

llvm-svn: 304129