When loading a module with no local entities, still bump the size of the

tables that correspond to ContinuousRangeMaps, since the keys to those
maps need to be unique, or we may map to the wrong offset.

This fixes a crash + malformed AST file seen when loading some modules
that import Cocoa on Darwin, which is a module with no contents except
imports of other modules. Unfortunately I have not been able to find a
reduced test case that reproduces this problem.

Also add an assert that we aren't mapping one key to multiple values
in CRM.  We ought to be able to say there are no duplicate keys at all,
but there are a bunch of 0 -> 0 mappings that are showing up, probably
coming from the source location table.