Add syntax example for <<__Native>> functions

`function foo();` is no longer valid - except if there's a
`<<__Native>>`, so it's no longer in Hack or HHVM's unit tests. It's
needed for generating docs.hhvm.com though

While I'm here:
- it's invalid hack outside of builtins, so make the typechecker ignore
all SyntaxExample files. we only need them to parser, we don't need them
to typecheck
- similarly, don't check for `hhvm -l` errors on syntax examples; if
  we've explicitly added them, we assume they're wanted, even if
 `hhvm -l` complains
- remove other example that no longer parses at all

.hhconfig

@@ -1,4 +1,4 @@
-ignored_paths = [ "tests/examples/NonHackFileTest/php_files/*", "vendor/bin" ]
+ignored_paths = [ "tests/examples/NonHackFileTest/php_files/.*", ".*SyntaxExample.*", "vendor/bin" ]
 error_php_lambdas = true
 disable_xhp_children_declarations = false
 allowed_decl_fixme_codes=2053,4045,4047

src/__Private/codegen/CodegenRelations.hack

@@ -55,7 +55,12 @@ final class CodegenRelations extends CodegenBase {
             }
           }
           if ($new) {
-            await execute_async('hhvm', '-l', $file);
+            // Check that examples are actually valid before including them in
+            // relations; some parse correctly, but are only valid at runtime
+            // in special circumstances, e.g. builtins
+            if (!(Str\contains($file, __FILE__) || Str\contains($file, 'SyntaxExample'))) {
+              await execute_async('hhvm', '-l', $file);
+            }
             foreach ($new as $key => $children) {
               $relationships->v[$key] =
                 Keyset\union($relationships->v[$key], $children);

src/__Private/codegen/data/PHPStaticAnonymousFunction.SyntaxExample → src/__Private/codegen/data/NativeFunctions.SyntaxExample.hack

@@ -7,6 +7,5 @@
  *
  */
 
-function php_anonymous_function_static(): void {
-  $_ = static function() {};
-}
+ <<__Native>>
+ function some_hhvm_builtin();