Don't include re-exports in a TU's imported module list.

We can get to these transitively; we should only record what the TU
actually claims to reference.

It turns out that we were still relying on this to force the load of
adapter modules for Clang modules. For now, we just force that up front,
even though currently that also forces the creation of ClangModule
wrappers for all transitive includes.

No intended visible functionality change.

Swift SVN r7012

lib/Serialization/ModuleFile.cpp

@@ -1340,6 +1340,7 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext,
       Module *baseModule = M;
       if (isWithinExtension) {
         baseModule = getModule(getIdentifier(rawAccessPath.front()));
+        assert(baseModule && "missing dependency");
         rawAccessPath = rawAccessPath.slice(1);
       }