[AST] Accept SynthesizedFileUnit in populateMemberCache

The differentiation and actor logics insert SynthesizedFileUnit in
SourceFile modules. Accepting these file units in populateMemberCache
allow to cache all the top-level decls of source file modules.

lib/AST/Module.cpp

@@ -276,8 +276,11 @@ void SourceLookupCache::populateMemberCache(const ModuleDecl &Mod) {
                              "populate-module-class-member-cache");
 
   for (const FileUnit *file : Mod.getFiles()) {
-    auto &SF = *cast<SourceFile>(file);
-    addToMemberCache(SF.getTopLevelDecls());
+    assert(isa<SourceFile>(file) ||
+           isa<SynthesizedFileUnit>(file));
+    SmallVector<Decl *, 8> decls;
+    file->getTopLevelDecls(decls);
+    addToMemberCache(decls);
   }
 
   MemberCachePopulated = true;