Allow non-static expressions in function parameter defaults

Summary: This is primarily for HNI expressions such as the following
in datetime:

  function date(string $format, int $now = TimeStamp::Current());

HHVM compiles default arguments to Bytecode and jumps to the correct
location in the function for initialization anyway, so there's no real
need to limit the default to static expressions apart from PHP's
language design.

Closes #2886

Reviewed By: @paulbiss

Differential Revision: D1682957

Signature: t1:1682957:1416007027:42343f4baf80906e22308e1e1e91bea7027967d1