Fill some gaps in .debug_loc.

Use best-effort guess when the location is unknown.  This is only
relevant for PCs in the middle of the statement where the debugger
should not usually stop in the first place.

The main motivation is to reduce the size .debug_loc since the
best-effort guesses allow us to merge consecutive entries.

Change-Id: I94bfd01363404e72a2c953309e59020b1a6a4764