Don't add ID mappings for offsets with no entities in a module

This is a better fix for 'duplicate key' problems in module continuous
range maps (vs what I added in r215810) by not adding any mappings at
all when there are no local entities. Now it also covers selectors,
which were not always being bumped because the record SELECTOR_OFFSET is
not always emitted.  I'll back out most of r215810 in a future commit,
since it should no longer be needed.

llvm-svn: 220207