Enable feature test macros

[cjdb]

clang-tidy doesn't seem to respect feature-test macros, but part of the design is done so that clang-tidy doesn't see through attempts to force a compile-time error (e.g. not using static_assert in the consteval function). This means that __cpp_lib_is_constant_evaluated can't be enabled till clang-tidy acknowledges that.

consteval doesn't have a feature-test macro at present.