[WebAssembly] Handle DebugLoc in DebugValueManager

According to
https://llvm.org/docs/HowToUpdateDebugInfo.html#when-to-preserve-an-instruction-location,
when moving (and in our case cloning) within the same BB, the debug
location is preserved. But when moving / cloning to a different BB, we
preserve the debug location only if the destination BB contains the same
location. Currently we preserve the debug loc unconditionally in all
cases. This CL correctly handles the debug locs in DebugValueManager.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D148115