NCGenerics: handle failure to load stdlib

The stdlib module can end up being a non-null ModuleDecl that contains
no files, due to a failure to load the stdlib.

In such an unusual case, fall-back to synthesizing Copyable/Escapable
into the Builtin module so that Sema can proceed.

rdar://129092011

Author: kavon

lib/AST/ASTContext.cpp

@@ -1313,6 +1313,14 @@ ASTContext::synthesizeInvertibleProtocolDecl(InvertibleProtocolKind ip) const {
     return proto;
 
   ModuleDecl *stdlib = getStdlibModule();
+  if (stdlib && stdlib->failedToLoad()) {
+    stdlib = nullptr; // Use the Builtin module instead.
+
+    // Ensure we emitted an error diagnostic!
+    if (!Diags.hadAnyError())
+      Diags.diagnose(SourceLoc(), diag::serialization_load_failed, "Swift");
+  }
+
   FileUnit *file = nullptr;
   if (stdlib) {
     file = &stdlib->getFiles()[0]->getOrCreateSynthesizedFile();