[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).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212788 91177308-0d34-0410-b5e6-96231b3b80d8