Fix is_expression and is_pattern_expression parsing #317
Description
The below two is expressions produce different parse trees. The former is correctly parsed as is_expression and latter is incorrectly parsed as is_pattern_expression.
var x = y is int;
var x = y is A;(compilation_unit [0, 0] - [1, 15]
(global_statement [0, 0] - [0, 17]
(local_declaration_statement [0, 0] - [0, 17]
(variable_declaration [0, 0] - [0, 16]
type: (implicit_type [0, 0] - [0, 3])
(variable_declarator [0, 4] - [0, 16]
(identifier [0, 4] - [0, 5])
(equals_value_clause [0, 6] - [0, 16]
(is_expression [0, 8] - [0, 16]
left: (identifier [0, 8] - [0, 9])
right: (predefined_type [0, 13] - [0, 16])))))))
(global_statement [1, 0] - [1, 15]
(local_declaration_statement [1, 0] - [1, 15]
(variable_declaration [1, 0] - [1, 14]
type: (implicit_type [1, 0] - [1, 3])
(variable_declarator [1, 4] - [1, 14]
(identifier [1, 4] - [1, 5])
(equals_value_clause [1, 6] - [1, 14]
(is_pattern_expression [1, 8] - [1, 14]
expression: (identifier [1, 8] - [1, 9])
pattern: (constant_pattern [1, 13] - [1, 14]
(identifier [1, 13] - [1, 14])))))))))