[Sema] Compiler should diagnose non-unique raw values without crashing

Enums with raw type `: String ` use the case name as the raw
value. So, two identical case names cause an identical raw value
error. Also, identical case names cause a separate redeclaration
error. The second error is diagnosed in a separate request in the
DeclVisitor. We may need a special case for this situation to
ignore the first error and defer diagnosing to the
CheckRedeclarationRequest. For now we will diagnose both errors.

`enum Foo : String {
    case Bar
    case Bar = "FooBar" // redclaration error only
    case Bar // redeclaration and raw value error
}`

Author: angela-laar

lib/Sema/TypeCheckDecl.cpp

@@ -1342,9 +1342,7 @@ EnumRawValuesRequest::evaluate(Evaluator &eval, EnumDecl *ED,
 
     // Diagnose the duplicate value.
     Diags.diagnose(diagLoc, diag::enum_raw_value_not_unique);
-    assert(lastExplicitValueElt &&
-           "should not be able to have non-unique raw values when "
-           "relying on autoincrement");
+    
     if (lastExplicitValueElt != elt &&
         valueKind == AutomaticEnumValueKind::Integer) {
       Diags.diagnose(uncheckedRawValueOf(lastExplicitValueElt)->getLoc(),
@@ -1356,6 +1354,7 @@ EnumRawValuesRequest::evaluate(Evaluator &eval, EnumDecl *ED,
     diagLoc = uncheckedRawValueOf(foundElt)->isImplicit()
         ? foundElt->getLoc() : uncheckedRawValueOf(foundElt)->getLoc();
     Diags.diagnose(diagLoc, diag::enum_raw_value_used_here);
+    
     if (foundElt != prevSource.lastExplicitValueElt &&
         valueKind == AutomaticEnumValueKind::Integer) {
       if (prevSource.lastExplicitValueElt)

test/Parse/enum.swift

@@ -364,6 +364,13 @@ enum DuplicateMembers7 : String { // expected-error {{'DuplicateMembers7' declar
   case Foo = "Bar" // expected-error {{invalid redeclaration of 'Foo'}}
 }
 
+enum DuplicateMembers8 : String { // expected-error {{'DuplicateMembers8' declares raw type 'String', but does not conform to RawRepresentable and conformance could not be synthesized}}
+  case Foo // expected-note {{'Foo' previously declared here}}
+  // expected-note@-1 {{raw value previously used here}}
+  case Foo // expected-error {{invalid redeclaration of 'Foo'}}
+  // expected-error@-1 {{raw value for enum case is not unique}}
+}
+
 // Refs to duplicated enum cases shouldn't crash the compiler.
 // rdar://problem/20922401
 func check20922401() -> String {