[Frontend] Enforce an error was emitted for invalid conformance

Make sure we emitted an error if we encountered an invalid conformance
error. This is important since `ASTContext::hadError` accounts for
such delayed errors, so we need to make sure we don't ever exit with
a non-zero exit code without emitting any error.

lib/FrontendTool/FrontendTool.cpp

@@ -1120,6 +1120,13 @@ static void performEndOfPipelineActions(CompilerInstance &Instance) {
 
   // Emit extracted constant values for every file in the batch
   emitConstValuesForAllPrimaryInputsIfNeeded(Instance);
+
+  // Make sure we emitted an error if we encountered an invalid conformance.
+  // This is important since `ASTContext::hadError` accounts for delayed
+  // conformance diags, so we need to ensure we don't exit with a non-zero exit
+  // code without emitting any error.
+  ASSERT(ctx.Diags.hadAnyError() || !ctx.hasDelayedConformanceErrors() &&
+         "Encountered invalid conformance without emitting error?");
 }
 
 static bool printSwiftVersion(const CompilerInvocation &Invocation) {