Matching the function-homing support for modular codegen. Any type
implicitly (implicit template specializations) or explicitly defined in
a module is attached to that module's object file and omitted elsewhere
(only a declaration used if necessary for references).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299987 91177308-0d34-0410-b5e6-96231b3b80d8