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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220207 91177308-0d34-0410-b5e6-96231b3b80d8
Revert most of r215810, which is no longer needed

Now that we no longer add mappings when there are no local entities,
there is no need to always bump the size of the tables that correspond
to ContinuousRangeMaps.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220208 91177308-0d34-0410-b5e6-96231b3b80d8