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.

compiler/rustc_interface/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(box_patterns)]
+#![feature(decl_macro)]
 #![feature(internal_output_capture)]
 #![feature(thread_spawn_unchecked)]
 #![feature(once_cell)]