Sema: Fix crash in MissingConformanceFailure::diagnoseAsError()

It's difficult to trigger this because the code path is only
reached for the standard operators. The issue in question was
in embedded Swift due to an unsupported usage of the ===
operator, but we expect an upcoming standard library change to
actually make us accept that code.

However, the fix should be pretty safe, even without a test case.

Fixes the crash in rdar://156095800.

Author: slavapestov

lib/Sema/CSDiagnostics.cpp

@@ -671,15 +671,17 @@ bool MissingConformanceFailure::diagnoseAsError() {
     };
 
     // Limit this to `Equatable` and `Comparable` protocols for now.
-    auto *protocol = getRHS()->castTo<ProtocolType>()->getDecl();
-    if (isEnumWithAssociatedValues(getLHS()) &&
-        (protocol->isSpecificProtocol(KnownProtocolKind::Equatable) ||
-         protocol->isSpecificProtocol(KnownProtocolKind::Comparable))) {
-      if (RequirementFailure::diagnoseAsError()) {
-        auto opName = getOperatorName(expr);
-        emitDiagnostic(diag::no_binary_op_overload_for_enum_with_payload,
-                       opName->str());
-        return true;
+    if (auto *protocolTy = getRHS()->getAs<ProtocolType>()) {
+      auto *protocol = protocolTy->getDecl();
+      if (isEnumWithAssociatedValues(getLHS()) &&
+          (protocol->isSpecificProtocol(KnownProtocolKind::Equatable) ||
+           protocol->isSpecificProtocol(KnownProtocolKind::Comparable))) {
+        if (RequirementFailure::diagnoseAsError()) {
+          auto opName = getOperatorName(expr);
+          emitDiagnostic(diag::no_binary_op_overload_for_enum_with_payload,
+                         opName->str());
+          return true;
+        }
       }
     }
   }