No longer emit nullable for typeconst param initialized to null

No longer emit nullable for typeconst param initialized to null

Summary:
Previously, hhbc and hhvm differed in the bytecode emitted for typeconst params that were set by default to null. For instance:

  <?hh // strict

  class TestClass {

    const type NullableString = ?string;

    public function f(TestClass::NullableString $a = null): void {
    }
  }

Previously, hhbc would label the param as `nullable hh_type extended_hint type_constant` but hhvm would only label it as `hh_type extended_hint type_constant`. Thus, we introduce a slight regression into hhbc in order to address this.

Reviewed By: michaeltingley

Differential Revision: D5913123

fbshipit-source-id: 59efe650211ffa7d44c9891a0ba4760426cf49ab