Variables/return values with constexpr type cannot have multiple
assignments. We check this now.
For conditionals, it is important to always infer a non-constexpr type.
This CL adds the ability to map any type (including union types) to be
mapped to their non-constexpr variant. Conditionals infer their type as
the non-constexpr version of a combination of the two branch types.

In addition, this improves subtyping for constexpr types:
If A extends B, then constexpr A extends constexpr B.
This makes it necessary to clean up "constexpr String", which has nothing
to do with tagged values.

Bug: v8:7793
Change-Id: Ia4d3cd5dc98f45b0ec89adf05c5c6111a0e51cc6
Reviewed-on: https://chromium-review.googlesource.com/1122864
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54167}