This CL adds grammar support for function pointers to generic builtins.
It also instantiates generic specializations when they are only used
in an assignment to a function pointer.

Example:

builtin GenericBuiltinTest<T: type>(c: Context, param: T): Object {
  return Null;
}

let fnptr: builtin(Context, Smi) => Object = GenericBuiltinTest<Smi>;

Change-Id: Ib7e5f47ffc05f14eb5d0b789936587263dfb961d
Reviewed-on: https://chromium-review.googlesource.com/1068731
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53284}