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.

Author: jyn514

src/debuginfo/mod.rs

@@ -59,7 +59,7 @@ impl DebugContext {
 
         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