Adding type variables declaration for methods

Summary:
Up until now a function could have type variables:
function foo<T>( ...)
but we want this to work with methods too

Test Plan: unit test

Reviewers: pad

Differential Revision: https://phabricator.fb.com/D493001

lang_php/parsing/parser_php.mly

@@ -703,14 +703,14 @@ class_statement:
      { UseTrait ($1, $2, Right ($3, $4, $5)) }
 
 method_declaration: 
-     method_modifiers T_FUNCTION is_reference method_name
+     method_modifiers T_FUNCTION is_reference method_name type_params_opt
      TOPAR parameter_list TCPAR
      return_type_opt
      method_body 
      { 
        Method {
          m_modifiers = $1; m_tok = $2; m_ref = $3; m_name = Name $4;
-         m_params = ($5, $6, $7); m_return_type = $8; m_body = $9;
+         m_params = ($6, $7, $8); m_return_type = $9; m_body = $10;
        }
      }
 

lang_php/parsing/unit_parsing_php.ml

@@ -166,6 +166,7 @@ let unittest =
       t "class A { const ?A<T1, T2> X = 0; }";
       t "$x = function(): ?int { return null; };";
       t "function foo(A<A<int>> $x): ?int { return null; };";
+      t "class A { public static function foo<T>(): ?int { } }";
     );
 
     (*-----------------------------------------------------------------------*)