[TBDGen] Fix check for global accessors (#18883)

Previously, TBDGen skipped emitting lazy initializers for globals that
appeared in any file with an entry point. This breaks, however on files
that have an NSApplicationMain/UIApplicationMain class in them, where
the entry point is synthesized but top-level globals are not locally
scoped. This change re-uses SILGen's check and only skips variable
declarations that appear at top level in a script mode file.

Resolves rdar://43549749

lib/AST/Decl.cpp

@@ -4481,6 +4481,27 @@ bool VarDecl::isSettable(const DeclContext *UseDC,
   return false;
 }
 
+bool VarDecl::isLazilyInitializedGlobal() const {
+  assert(!getDeclContext()->isLocalContext() &&
+         "not a global variable!");
+  assert(hasStorage() && "not a stored global variable!");
+
+  // Imports from C are never lazily initialized.
+  if (hasClangNode())
+    return false;
+
+  if (isDebuggerVar())
+    return false;
+
+  // Top-level global variables in the main source file and in the REPL are not
+  // lazily initialized.
+  auto sourceFileContext = dyn_cast<SourceFile>(getDeclContext());
+  if (!sourceFileContext)
+    return true;
+
+  return !sourceFileContext->isScriptMode();
+}
+
 bool SubscriptDecl::isSettable() const {
   return supportsMutation();
 }