[Patches] Add variant constexpr support for visit, comparisons and get

[Tim Shen]

This 4-patch series contains the following in order:

a.diff: Remove uses-allocator ctors. They are going away, and removing
it reduces the maintenance burden from now on.

b.diff: Add constexpr support for get<> and comparisons. This patch
also involves small refactoring of _Variant_storage.

c.diff: Fix some libc++ test failures.

d.diff: Add constexpr support for visit. This patch also removes
__storage, __get_alternative, and __reserved_type_map, since we don't
need to support reference/void types for now.

The underlying design doesn't change - we still use the vtable
approach to achieve O(1) runtime cost even under -O0.

Bootstrapped and tested for each of them.

Thanks!


-- 
Regards,
Tim Shen
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