Use Clang's `__is_trivially_equality_comparable` intrinsic to optimize more cases in vector algorithms

[AlexGuteniev]

__is_trivially_equality_comparable is a trait that is true if a type is memcmp comarable for == and != against itself. We can use that on Clang along with existing metaprogramming.

This will allow covering more types like struct containing only integers with defaulted comparison and enums without user-defined comparison. This includes user defined types, for which this cannot be achieved without "magic". Also this includes standard types, such as chrono::duration or pair<int, int> if their comparison is defaulted properly; this should be less burden that specializing traits for them all.

A separate escape hatch could be a good idea, due to quasi-supporting old Clang and "new" Intel, which may not have __is_trivially_equality_comparable, or may have more bugs in it.

These algorithms can be improved:

• equal for all sizes
• search, find_end, adjacent_find, find_first_of, search_n, unique, unique_copy for vector element sizes
• mismatch for all sizes
• find, find_last, remove, remove_copy, count for vector element sizes
• replace for some vector element sizes (32-bit and 64-bit)

[StephanTLavavej]

Clang supports __has_builtin which is an effective way to automatically light up support. I probably should have used it instead of version checks recently.

[AlexGuteniev]

Clang supports __has_builtin which is an effective way to automatically light up support.

I want to enable that only starting with Clang 19, since I fear the bugs of early implementation:

• __is_trivially_equality_comparable gives wrong answer when inheritance is involved llvm/llvm-project#63192
• std::equal gives wrong answer on array of arrays llvm/llvm-project#63371
• __is_trivially_equality_comparable gives wrong answer when arrays are involved llvm/llvm-project#63656
• __is_trivially_equality_comparable(T) false positive when the defaulted operator is ineligible llvm/llvm-project#89293
• After c6e996a931f, Assertion failed: (Loc.isValid() && "point of instantiation must be valid!"), function setPointOfInstantiation llvm/llvm-project#107777