[Frontend] Diagnose rather than exit/assert on experimental features

Diagnose rather than exit when using experimental features that cannot
be enabled in production. Also diagnose if using a compiler without
`SwiftCompilerSources` when enabling embedded.

This is especially important for long-running services like sourcekitd,
which shouldn't exit just because an invalid argument was passed.

Resolves rdar://119724074.

Author: bnbarham

include/swift/AST/DiagnosticsFrontend.def

@@ -563,6 +563,12 @@ ERROR(objc_with_embedded,none,
       "Objective-C interoperability cannot be enabled with embedded Swift.", ())
 ERROR(no_allocations_without_embedded,none,
       "-no-allocations is only applicable with embedded Swift.", ())
+ERROR(no_swift_sources_with_embedded,none,
+      "embedded swift cannot be enabled in a compiler built without Swift sources", ())
+
+ERROR(experimental_not_supported_in_production,none,
+      "experimental feature '%0' cannot be enabled in production compiler",
+      (StringRef))
 
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Frontend/CompilerInvocation.cpp

@@ -845,13 +845,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     if (auto feature = getExperimentalFeature(value)) {
 #ifdef NDEBUG
       if (!isFeatureAvailableInProduction(*feature)) {
-        llvm::errs() << "error: experimental feature '" << A->getValue() 
-                     << "' cannot be enabled in a production compiler\n";
-        exit(1);
+        Diags.diagnose(SourceLoc(), diag::experimental_not_supported_in_production,
+                       A->getValue());
+        HadError = true;
+      } else {
+        Opts.Features.insert(*feature);
       }
-#endif
-
+#else
       Opts.Features.insert(*feature);
+#endif
     }
 
     // Hack: In order to support using availability macros in SPM packages, we
@@ -1368,12 +1370,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.BypassResilienceChecks |= Args.hasArg(OPT_bypass_resilience);
 
   if (Opts.hasFeature(Feature::Embedded)) {
-    assert(swiftModulesInitialized() && "no SwiftCompilerSources");
-
     Opts.UnavailableDeclOptimizationMode = UnavailableDeclOptimization::Complete;
     Opts.DisableImplicitStringProcessingModuleImport = true;
     Opts.DisableImplicitConcurrencyModuleImport = true;
 
+    if (!swiftModulesInitialized()) {
+      Diags.diagnose(SourceLoc(), diag::no_swift_sources_with_embedded);
+      HadError = true;
+    }
+
     if (FrontendOpts.EnableLibraryEvolution) {
       Diags.diagnose(SourceLoc(), diag::evolution_with_embedded);
       HadError = true;