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GH-15098: [C++] fix util::EqualityComparable to compile on clang 15 (#33940)

If you have an equality operator then C++20 then clang 15 expects it to be symmetric.  The util::EqualityComparable operator== was defined as:

`bool operator==(util::EqualityComparable<T>, T)`

and there is no equivalent:

`bool operator==(T, util::EqualityComparable<T>)`

This was later determined to be unintentional strictness and the [standard was clarified](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2468r2.html).  This error was relaxed in clang 16.  Equality only needs to be symmetric if you want the compiler to provide `!=` for you (which we don't rely on).  However, it seems like a worthwhile fix, and it doesn't appear that Clang will be backporting the fix to clang 15 (that is my interpretation of the fact that clang lists the defect as clang-16 in this conformance page: https://clang.llvm.org/cxx_status.html).

Furthermore, the old definition allowed for `Scalar::operator==(std::shared_ptr<Scalar> other)` (i.e. comparing with a shared_ptr) and it isn't clear to me that should be allowed (it's generally inconsistent with the rest of the code base).  This change seems to have undone that and I made no change to restore it (and instead fixed the references to that odd equality operation).

BREAKING CHANGE: The functions `bool Scalar::Equals(const std::shared_ptr<Scalar>&) const` and `bool Scalar::Equals(const std::shared_ptr<Scalar>&, const  EqualOptions&) const` have been removed.  Instead `bool Scalar::Equals(const Scalar&) const` and `bool Scalar::Equals(const Scalar&, const EqualOptions&) const` should be used.

BREAKING CHANGE: `FileInfo::ToString` now includes the size and mtime.

* Closes: #15098

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>