Fix logic fail and unsets in DefineConstant()

If we infer the type, then we already know
its integer type ID.  Trying to then parse that type
leads to "Undefined type" errors (but it still functions
due to the fallthrough).

This diff skips trying to parse a known type and thus
avoids the misleading error message.

Also guard against missing funcs/consts elements in
global scope and classes.

hphp/idl/base.php

@@ -280,12 +280,13 @@ function idl_infer_type($v) {
 
 function DefineConstant($const) {
   global $constants, $classes, $current_class;
+
   if (!isset($const['type']) && array_key_exists('value', $const)) {
     $const['type'] = idl_infer_type($const['value']);
+  } else {
+    $const['type'] = idl_parse_type($const['type']);
   }
 
-  $const['type'] = idl_parse_type($const['type']);
-
   if (empty($current_class)) {
     $constants[] = $const;
   } else {
@@ -370,17 +371,21 @@ function DefineFunction($func) {
 function ReadIDLFile($path) {
   $entries = json_decode(file_get_contents($path), /* use arrays */ true);
 
-  foreach ($entries['funcs'] as $func) {
-    DefineFunction($func);
+  if (!empty($entries['funcs'])) {
+    foreach ($entries['funcs'] as $func) {
+      DefineFunction($func);
+    }
   }
 
-  foreach ($entries['consts'] as $const) {
-    DefineConstant($const);
+  if (!empty($entries['consts'])) {
+    foreach ($entries['consts'] as $const) {
+      DefineConstant($const);
+    }
   }
 
   foreach ($entries['classes'] as $class) {
-    $methods = $class['funcs'];
-    $consts = $class['consts'];
+    $methods = isset($class['funcs']) ? $class['funcs'] : array();
+    $consts = isset($class['consts']) ? $class['consts'] : array();
     unset($class['funcs']);
     unset($class['consts']);