This CL adds constants that can be defined in the module scope:

const kConstexprConst: constexpr int31 = 5;
const kIntptrConst: intptr = 4;
const kSmiConst: Smi = 3;

They are implemented by generating "mini-macros" that return the
expression on the right-hand side of the assignment.

Bug: v8:7793
Change-Id: I0a476cb3111707fad56bf15e9547b377c7adab37
Reviewed-on: https://chromium-review.googlesource.com/1114745
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54430}