[modules] Rework serialized DeclContext lookup table management. Instead of

walking the loaded ModuleFiles looking for lookup tables for the context, store
them all in one place, and merge them together if we find we have too many
(currently, more than 4). If we do merge, include the merged form in our
serialized lookup table, so that downstream readers never need to look at our
imports' tables.

This gives a huge performance improvement to builds with very large numbers of
modules (in some cases, more than a 2x speedup was observed).

llvm-svn: 246497