fix -typecheck -verify and experimental features

When using `-enable-experimental-feature` on a non-asserts build,
we only emit an error diagnostic that has no source-line information
and continue to enable the feature.

That doesn't actually prevent use of the experimental feature when
you are passing `-typecheck -verify`, since in diagnostics verification
mode, a diagnostic with an unknown error location is ignored. Thus,
the experimental feature is enabled and run for type-checking, but
the compiler would exit with a zero error code.

This patch takes a hammer to that escape-hatch, forcing an early
non-zero exit the moment an experimental feature is requested. The
error message is output to stderr so that CI and other tools should see
what happened.

lib/Frontend/CompilerInvocation.cpp

@@ -812,9 +812,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     if (auto feature = getExperimentalFeature(value)) {
 #ifdef NDEBUG
       if (!isFeatureAvailableInProduction(*feature)) {
-        Diags.diagnose(SourceLoc(),
-                       diag::error_experimental_feature_not_available,
-                       A->getValue());
+        llvm::errs() << "error: experimental feature '" << A->getValue() 
+                     << "' cannot be enabled in a production compiler\n";
+        exit(1);
       }
 #endif