No they cannot, clangd isn't compiler agnostic, and Clion's dependency on it is now being hindered by clang's slow pace, even if the goal is to target another compiler.
To be fair, from a language point of view, clang is not actually lagging that badly. I don't know how reliable is this: https://en.cppreference.com/w/cpp/compiler_support, but language-wise clang is sort of keeping up.
libc++ is indeed in a sorry state, but at least on linux clang is usually used against libstdc++ so it is not a huge deal. Personally I'm worried about clangd falling behind, but so far I see very very little that it doesn't understand.
No they cannot, clangd isn't compiler agnostic, and Clion's dependency on it is now being hindered by clang's slow pace, even if the goal is to target another compiler.