This CL adds the new type expression
builtin(Context, ArgType1, ...) => ReturnType
and allows to use Torque-defined builtins as values of this type, as well
as calling values of this type.
The new function pointer types are subtypes of Code.

Change-Id: Ib7ba3ce6ef7a8591a4c79230dd189fd25698d5b9
Reviewed-on: https://chromium-review.googlesource.com/1060056
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53217}