[Frontend] Set "failed to load" bit if stdlib fails

Set the "failed to load" flag on the main module
if the stdlib fails to load, as we'll bail without
adding any source files to the module.

Author: hamishknight

lib/Frontend/Frontend.cpp

@@ -744,6 +744,10 @@ void CompilerInstance::performSemaUpTo(SourceFile::ASTStage_t LimitStage,
   if (LimitStage > SourceFile::Unprocessed &&
       Invocation.getImplicitStdlibKind() == ImplicitStdlibKind::Stdlib
       && !loadStdlib()) {
+    // If we failed to load the stdlib, mark the main module as having
+    // "failed to load", as it will contain no files.
+    // FIXME: We need to better handle a missing stdlib.
+    mainModule->setFailedToLoad();
     return;
   }