This CL adds local const bindings. This means that instead of
generating TVARIABLEs for variables, we can generate simple TNodes.

Example:

macro FooBar(): {
  const kSomeSmi: Smi = 10;
  ...
}

This CL also enforces that variables with a constexpr type are bound
using 'const' and not 'let'.

R=tebbi@chromium.org

Bug: v8:7793
Change-Id: Id20a18149df9fc374ce718bdb1478e3eabb6e6df
Reviewed-on: https://chromium-review.googlesource.com/1138316
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54479}