[AST] Return whether type computation of EnumElementDecl succeeded

This is no change in functionality, but allows for easier bailing out when
type-checking EnumElementDecls.

Author: JaSpa

include/swift/AST/Decl.h

@@ -5270,7 +5270,9 @@ class EnumElementDecl : public ValueDecl {
         static_cast<unsigned>(ElementRecursiveness::NotRecursive);
   }
 
-  void computeType();
+  /// \returns false if there was an error during the computation rendering the
+  /// EnumElementDecl invalid, true otherwise.
+  bool computeType();
 
   bool hasArgumentType() const { return !ArgumentType.getType().isNull(); }
   Type getArgumentType() const { return ArgumentType.getType(); }

lib/AST/Decl.cpp

@@ -4309,7 +4309,7 @@ SourceRange EnumElementDecl::getSourceRange() const {
   return {getStartLoc(), getNameLoc()};
 }
 
-void EnumElementDecl::computeType() {
+bool EnumElementDecl::computeType() {
   EnumDecl *ED = getParentEnum();
 
   Type resultTy = ED->getDeclaredTypeInContext();
@@ -4326,6 +4326,7 @@ void EnumElementDecl::computeType() {
     resultTy = FunctionType::get(argTy, resultTy);
 
   setType(resultTy);
+  return true;
 }
 
 Type EnumElementDecl::getArgumentInterfaceType() const {

lib/Sema/TypeCheckDecl.cpp

@@ -5543,11 +5543,14 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       EED->setRecursiveness(ElementRecursiveness::NotRecursive);
     }
 
-    // Now that we have an argument type we can set the element's declared
-    // type.
-    if (!EED->hasType())
-      EED->computeType();
-    EED->setIsBeingTypeChecked(false);
+    {
+      defer { EED->setIsBeingTypeChecked(false); };
+
+      // Now that we have an argument type we can set the element's declared
+      // type.
+      if (!EED->hasType() && !EED->computeType())
+        return;
+    }
 
     // Test for type parameters, as opposed to a generic decl context, in
     // case the enclosing enum type was illegally declared inside of a generic