AST: Early exit from subst() if the type is concrete

If the type has no archetypes or type parameters there's no
reason to walk through it because it will not change.

Author: slavapestov

lib/AST/Type.cpp

@@ -2907,7 +2907,16 @@ static Type substType(
     llvm::PointerUnion<ModuleDecl *, const SubstitutionMap *> conformances,
     TypeSubstitutionFn substitutions,
     SubstOptions options) {
+
+  // FIXME: Change getTypeOfMember() to not pass GenericFunctionType here
+  if (!derivedType->hasArchetype() &&
+      !derivedType->hasTypeParameter() &&
+      !derivedType->is<GenericFunctionType>())
+    return derivedType;
+
   return derivedType.transform([&](Type type) -> Type {
+    // FIXME: Add SIL versions of mapTypeIntoContext() and
+    // mapTypeOutOfContext() and use them appropriately
     assert((options.contains(SubstFlags::AllowLoweredTypes) ||
             !isa<SILFunctionType>(type.getPointer())) &&
            "should not be doing AST type-substitution on a lowered SIL type;"