[modules] Be sure to emit local specializations of imported templates, even if

[modules] Be sure to emit local specializations of imported templates, even if
the resulting specialization is not referenced by the rest of the AST. This
both avoids performing unnecessary reinstantiations in downstream users of the
AST file and fixes a bug (breaking modules self-host right now) where we would
sometimes fail to emit a definition of a class template specialization if we
imported just a declaration of it from elsewhere (see new testcase for reduced
example).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283489 91177308-0d34-0410-b5e6-96231b3b80d8