[modules] Enforce the rules that an explicit or partial specialization must be

[modules] Enforce the rules that an explicit or partial specialization must be
declared before it is used. Because we don't use normal name lookup to find
these, the normal code to filter out non-visible names from name lookup results
does not apply.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268585 91177308-0d34-0410-b5e6-96231b3b80d8