In what sense? Apple keeps Clang up to date on macOS, and it is essentially no work to get the associated Clang tooling and other C++ tools (valgrind, etc) installed through something like homebrew.
Actually, not especially. The system compiler that ships with Xcode is still quite old and the libraries are quite out of date. For example, though the most recent Xcode finally added -std=c++17 it still lacks core library features like std::variant. I wish they could keep close to mainline but for whatever reason they can't.
quite old, quite old... -std=c++17 was only added in clang 5 released in september, 3 months ago. AFAIK current Xcode is based on Clang 4.
As far as compiler support goes, Clang supports all of C++17 except for P0522R0 which is a defect report (ambiguity in specification) so cannot be implemented as written. Ditto GCC.
However std::variant is a library entity, not a compiler entity, and indeed, the libraries are a bit behind. Neither GCC's nor LLVM's runtime library support the parallelism extensions and have other gaps in areas exotic (memory, unfortunately which I would like to use) and prosaic (filesystem, which though I'd like to use it as well don't really care about). But they have been solid on C++17 core functionality for quite a long time.
C++ isn't as important to Apple as Swift (or even, still, ObjC) and C++17 is pretty new, so one can hardly blame Apple for this, especially since it's so trivial to simply use another compiler.
It's due to issues like this that I do all our clang testing on FreeBSD, where it's a more vanilla build of the llvm codebase, and I treat AppleClang as a wholly separate compiler variant which requires independent testing.