[modules] Don't try to merge template specializations by performing name lookup

[modules] Don't try to merge template specializations by performing name lookup
into their container; we won't find them there. These things are already being
merged when they're added to their primary template's folding set, so this
merging is redundant (and causes us to reject-valid because we think we've
found an odr violation).

llvm-svn: 212788