Fix a SourceKitCrash in the VTableSpecializer pass

Replace the assert-check if a vtable is available with a regular error message.
This cannot occur in regular builds - only if built with embedded swift and without wmo.
The command line compiler prevents this combination, but it  can happen in SourceKit.

rdar://130167087

lib/SILOptimizer/Transforms/VTableSpecializer.cpp

@@ -165,9 +165,12 @@ SILVTable *swift::specializeVTableForType(SILType classTy, SILModule &module,
   ClassDecl *classDecl = genClassTy->getDecl();
   SILVTable *origVtable = module.lookUpVTable(classDecl);
   if (!origVtable) {
-    llvm::errs() << "No vtable available for "
-                 << genClassTy->getDecl()->getName() << '\n';
-    llvm::report_fatal_error("no vtable");
+    // This cannot occur in regular builds - only if built without wmo, which
+    // can only happen in SourceKit.
+    // Not ideal, but better than a SourceKit crash.
+    module.getASTContext().Diags.diagnose(
+        SourceLoc(), diag::cannot_specialize_class, classTy.getASTType());
+    return nullptr;
   }
 
   SubstitutionMap subs = astType->getContextSubstitutionMap(