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
(cherry picked from commit cbb6e69)

Author: kavon

include/swift/AST/Module.h

@@ -387,6 +387,10 @@ class ModuleDecl
   /// Only to be called by MemoryBufferSerializedModuleLoader.
   void setBypassResilience() { BypassResilience = true; }
 
+  bool hasFiles() const {
+    return !Files.empty();
+  }
+
   ArrayRef<FileUnit *> getFiles() {
     assert(!Files.empty() || failedToLoad());
     return Files;

lib/AST/ASTContext.cpp

@@ -1256,7 +1256,7 @@ ASTContext::synthesizeInvertibleProtocolDecl(InvertibleProtocolKind ip) const {
 
   ModuleDecl *stdlib = getStdlibModule();
   FileUnit *file = nullptr;
-  if (stdlib) {
+  if (stdlib && stdlib->hasFiles()) {
     file = &stdlib->getFiles()[0]->getOrCreateSynthesizedFile();
   } else {
     file = &TheBuiltinModule->getMainFile(FileUnitKind::Builtin);
@@ -1280,7 +1280,7 @@ ASTContext::synthesizeInvertibleProtocolDecl(InvertibleProtocolKind ip) const {
   protocol->setAccess(AccessLevel::Public);
 
   // Hack to get name lookup to work after synthesizing it into the stdlib.
-  if (stdlib) {
+  if (stdlib && stdlib->hasFiles()) {
     cast<SynthesizedFileUnit>(file)->addTopLevelDecl(protocol);
     stdlib->clearLookupCache();
   }