Fix HPHP's parser when handling generics

We need to allow any type after the AS keyword, including xhp class names.

hphp/test/quick/hh_type_args4.php

@@ -8,5 +8,8 @@ class B<T> extends A<T> { }
 function foo<T, Q as A<T> >(Q $thing, T $item): T {
   return $thing->getT($item);
 }
+class C<T as A<int>> {}
+class D<T as :x:base> {}
+class E<T as A<:x:base>> {}
 echo "Done\n";
 

hphp/util/parser/hphp.ll

@@ -113,6 +113,7 @@ static int getNextTokenType(int t) {
     case T_BOOL_CAST:
     case T_UNSET_CAST:
     case T_UNRESOLVED_LT:
+    case T_AS:
       return NextTokenType::XhpTag |
              NextTokenType::XhpClassName;
     case ',': case '(': case '|':