AST: Fix crash with circular raw value

Fixes rdar://problem/123543175.

Author: slavapestov

lib/AST/Decl.cpp

@@ -5634,7 +5634,8 @@ EnumDecl::EnumDecl(SourceLoc EnumLoc,
 Type EnumDecl::getRawType() const {
   ASTContext &ctx = getASTContext();
   return evaluateOrDefault(
-      ctx.evaluator, EnumRawTypeRequest{const_cast<EnumDecl *>(this)}, Type());
+      ctx.evaluator, EnumRawTypeRequest{const_cast<EnumDecl *>(this)},
+      ErrorType::get(ctx));
 }
 
 void EnumDecl::setRawType(Type rawType) {

validation-test/compiler_crashers_2_fixed/rdar123543175.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift
+
+public enum E : E.R {
+// expected-error@-1 {{'E' has a raw type that depends on itself}}
+// expected-note@-2 2{{through reference here}}
+// expected-note@-3 {{while resolving type 'E.R'}}
+// expected-error@-4 {{'R' is not a member type of enum 'rdar123543175.E'}}
+// expected-note@-5 {{'E' declared here}}
+  case x
+}