[AutoDiff upstream] Add common SIL differentiation utilities.

lib/AST/Module.cpp

@@ -2232,6 +2232,11 @@ SourceFile::getCachedVisibleDecls() const {
   return getCache().AllVisibleValues;
 }
 
+void SourceFile::addVisibleDecl(ValueDecl *decl) {
+  Decls->push_back(decl);
+  getCache().AllVisibleValues.push_back(decl);
+}
+
 static void performAutoImport(
     SourceFile &SF,
     SourceFile::ImplicitModuleImportKind implicitModuleImportKind) {