Remove all attempts to use Clang builtins for trivial relocatability #1985
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This simplifies the codepaths after a4950fb. We can't use Clang's current `__builtin_is_cpp_trivially_relocatable(T)` for the same reason we continue to be unable to use Clang's current `__is_trivially_relocatable(T)`: it returns true for polymorphic types and other types that aren't trivially relocatable in the Abseil/Folly/P1144 sense. We use memcpy for relocation; we use relocation to implement InlineVector::erase and InlineVector::swap. So, as the comment says, we care about wonky assignment as well as wonky copy-construction or destruction. Use the public (not private/compiler-builtin!) API of P1144 wherever it's available. Never use the Clang builtins otherwise. Always fall back to the public (not private/compiler-builtin!) std::is_trivially_copyable when P1144 isn't available. Remove references to P2786; it is not part of C++26 anymore, and even if it were, Abseil couldn't use it, because P2786 ignored move-assignment and also permitted "relocation" to fix up vptrs (which memcpy never does).
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This simplifies the codepaths after a4950fb. We can't use Clang's current
__builtin_is_cpp_trivially_relocatable(T)for the same reason we continue to be unable to use Clang's current__is_trivially_relocatable(T): it returns true for polymorphic types and other types that aren't trivially relocatable in the Abseil/Folly/P1144 sense. We use memcpy for relocation; we use relocation to implement InlineVector::erase and InlineVector::swap. So, as the comment says, we care about wonky assignment as well as wonky copy-construction or destruction.Use the public (not private/compiler-builtin!) API of P1144 wherever it's available. Never use the Clang builtins otherwise. Always fall back to the public (not private/compiler-builtin!) std::is_trivially_copyable when P1144 isn't available.
Remove references to P2786; it is not part of C++26 anymore, and even if it were, Abseil couldn't use it, because P2786 ignored move-assignment and also permitted "relocation" to fix up vptrs (which memcpy never does).
(Fixes #1885 in a different, shorter/correcter, way.)