message: Fix `rustdoc --version` when used with download-rustc
  Previously, rustdoc would unconditionally report the version that *rustc* was compiled with.
  That showed things like `nightly-2022-10-30`, which wasn't right, since this was a `dev` build compiled from source.
  Fix it by changing `rustc_driver::version` to a macro expanded at invocation time.
diff: @@ -59,7 +59,7 @@ pub(crate) fn new(tcx: TyCtxt<'_>, isa: &dyn TargetIsa) -> Self {
  let producer = format!(
  "cg_clif (rustc {}, cranelift {})",
  - rustc_interface::util::version_str().unwrap_or("unknown version"),
  + rustc_interface::util::rustc_version_str().unwrap_or("unknown version"),
  cranelift_codegen::VERSION,
  );
  let comp_dir = tcx
  