Revert "Revert "[Modules] Remove unnecessary check when generating name lookup table in ASTWriter""

This re-lands commit db987b95,
previously reverted in commit 1e070916,
with a fix for a pre-existing bug in how we write constructor names into
lookup tables that was exposed by that change.

As yet, we've been unable to produce a reduced testcase for this bug,
but we understand the problem: if we write out a lookup table for a
class which has two constructor names, and we only have local lookup
results for one of those constructor names, then we will write out a
lookup table where the "all the constructors" lookup result contains
results for only one of the constructor names.