[AST] Move the empty module assertion

Start asserting in `ModuleDecl::getFiles`
that the module is either non-empty or has
failed to load. This ensures that module
loading doesn't attempt to query the module's
files until the ModuleFile has been installed.

Author: hamishknight

include/swift/AST/Module.h

@@ -363,6 +363,7 @@ class ModuleDecl : public DeclContext, public TypeDecl {
   ArrayRef<ImplicitImport> getImplicitImports() const;
 
   ArrayRef<FileUnit *> getFiles() {
+    assert(!Files.empty() || failedToLoad());
     return Files;
   }
   ArrayRef<const FileUnit *> getFiles() const {

lib/AST/ASTContext.cpp

@@ -1515,11 +1515,6 @@ void ASTContext::verifyAllLoadedModules() const {
   FrontendStatsTracer tracer(Stats, "verify-all-loaded-modules");
   for (auto &loader : getImpl().ModuleLoaders)
     loader->verifyAllModules();
-
-  for (auto &topLevelModulePair : LoadedModules) {
-    ModuleDecl *M = topLevelModulePair.second;
-    assert(!M->getFiles().empty() || M->failedToLoad());
-  }
 #endif
 }