As it turns out, enums can be used to declare bitfields, and we
cannot form a reference to a bitfield in the cpature. Thus, we add
`std::is_enum` as a criteria to the default for `capture_by_value`,
so that enum-based bitfields are also captured by value and thus
decomposable.

Closes #3001

Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.

Closes #2854

The issue was that `capture_by_value` was meant to be specialized
by plain type, e.g. `capture_by_value<std::weak_ordering>`, but
the TMP in Decomposer did not properly throw away `const` (and
`volatile`) qualifiers, before taking the value of `capture_by_value`.

The Core Guidelines state "A base class destructor should be either
public and virtual, or protected and non-virtual" so, hopefully, no
static analyser will complain.

This was originally motivated by `REQUIRE((a <=> b) == 0)` no
longer compiling using MSVC. After some investigation, I found
that they changed their implementation of the zero literal
detector from the previous pointer-constructor with deleted
other constructors, into one that uses `consteval` constructor
from int.

This breaks the previous detection logic, because now
`is_foo_comparable<std::strong_ordering, int>` is true, but
actually trying to compare them is a compile-time error...
The solution was to make the decomposition `constexpr` and rely
on a late C++20 DR that makes it so that `consteval` propagates
up through the callstack of `constexpr` functions, until it either
runs out of `constexpr` functions, or succeeds.

However, the default handling of types in decomposition is to
take a reference to them. This reference never becomes dangling,
but because the constexpr evaluation engine cannot prove this,
decomposition paths taking references to objects cannot be
actually evaluated at compilation time. Thankfully we already
did have a value-oriented decomposition path for arithmetic types
(as these are common linkage-less types), so we could just
explicitly spell out the `std::foo_ordering` types as also being
supposed to be decomposed by-value.

Two more fun facts about these changes
 1) The original motivation of the MSVC change was to avoid
    trigering a `Wzero-as-null-pointer-constant` warning. I still
    do not believe this was a good decision.
 2) Current latest version of MSVC does not actually implement the
    aforementioned C++20 DR, so even with this commit, MSVC cannot
    compile `REQUIRE((a <=> b) == 0)`.