In the process:
  - Add strict ordering of Types so that name mangling is consistent
    and build time. Previously, the UnionType stored the union's
    types in a std::set<const Type*>, which did not have a consistent
    ordering of the types in the set.
  - Add a int31 type to enable consistency and correctness of
    handling of 'constexpr int31' values on the C++ side.
  - By removing the "implicit" keyword for operators, there is now
    one less difference between operators and calls, another
    incremental step in unifying operators and calls.
  - Enable external (i.e. C++-defined) generic specializations
  - Add CSA support for checking double ElementsKinds, including
    tests.
  - Clean up some constexpr/non-constexpr handling of ElementsKinds.

Bug: v8:7793
Change-Id: I27699aba70b98ebf5466e5b62b045d7b1dad62c8
Reviewed-on: https://chromium-review.googlesource.com/1091155
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53664}