[AST] Fail to compute the EnumElementDecl type in case of an ErrorType

If the resulting function type contains an ErrorType, this is an indicator that
something else is wrong. Bail out in this case.
This happened if an enum case was referenced inside the where-clause and treated
as a type.

Author: JaSpa

lib/AST/Decl.cpp

@@ -4311,8 +4311,13 @@ SourceRange EnumElementDecl::getSourceRange() const {
 
 bool EnumElementDecl::computeType() {
   EnumDecl *ED = getParentEnum();
-
   Type resultTy = ED->getDeclaredTypeInContext();
+
+  if (resultTy->is<ErrorType>()) {
+    setType(resultTy);
+    return false;
+  }
+
   Type argTy = MetatypeType::get(resultTy);
 
   // The type of the enum element is either (T) -> T or (T) -> ArgType -> T.

test/Generics/generic_types.swift

@@ -334,3 +334,5 @@ struct UnsolvableInheritance1<T : T.A> {}
 struct UnsolvableInheritance2<T : U.A, U : T.A> {}
 // expected-error@-1 {{inheritance from non-protocol, non-class type 'U.A'}}
 // expected-error@-2 {{inheritance from non-protocol, non-class type 'T.A'}}
+
+enum X7<T where X7.X : G> { case X } // expected-error{{'X' is not a member type of 'X7<T>'}}

validation-test/compiler_crashers/28224-swift-genericfunctiontype-get.swift renamed to validation-test/compiler_crashers_fixed/28224-swift-genericfunctiontype-get.swift

@@ -5,6 +5,6 @@
 // See http://swift.org/LICENSE.txt for license information
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -parse
+// RUN: not %target-swift-frontend %s -parse
 // REQUIRES: asserts
 {enum S<T where S.c:A{case c