Note: Not an expert.
First, Rust doesn’t use or depend on clang in any way. You’re confusing clang with the backend it uses: LLVM.
Second, you’re confusing LLVM supporting a platform as a host, and a platform as a target. As I understand it, XP will no longer be supported as a host (i.e. you can’t run LLVM on it), but will still be supported as a target (i.e. you can generate code to run on it).
Given that Firefox still has more people on XP than all versions of Linux put together, and Mozilla wants to use Rust components in Firefox, it’s highly likely that Rust will support targeting XP for a while.