Please add flag -fconcepts

[cjdb]

I'd like to integrate a clang-concepts-built clang-tidy into my CMake project, which means that it's dependent on GCC flags when building with GCC. Since GCC uses -fconcepts, this means that clang-tidy won't work due to an unknown flag.

It'd be nice if either of the following were satisfied.

• -fconcepts is equivalent to -Xclang -fconcepts-ts (which makes more sense, since clang-concepts targets C++20 concepts, not Concepts TS concepts)
• -fconcepts is ignored with a remark or warning to notify the user (less-preferred, but I think it's workable).

[davidstone]

It would be even nicer if -fconcepts and -fconcepts-ts were both ignored and this branch just accepted code with concepts without needing any special flags. Why is this behind a flag when at this point it's just another C++20 feature (and the only reason anyone would be using this branch)?