RFC: Increasing the GCC and Clang requirements to support C++17 in LLVM

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@tobiashieta

I guess one thing we should discuss is if the soft error should be part of LLVM14 or 15? If 15 then the first release we can use C++17 in is LLVM 16.

Making the soft error in LLVM 14 leaves too much uncertainty in the time window for people building trunk to get their builds changed.

@tobiashieta

Turn the soft-error into a hard-error after said LLVM release has branched.

… because LLVM 14 has already branched. It seems to me the question of asking for doing the soft error in LLVM 14 is really asking to retroactively decide that trunk should hard error immediately.