Simplify the current_rustc_version macro.

It currently has the syntax
`current_rustc_version!(env!("CFG_RELEASE"))` where the
`env!("CFG_RELEASE")` part looks like a normal expression but it is
actually parsed and processed by the `current_rustc_version` macro.

The documented rationale for this is that you'll find it if you grep for
`env!("CFG_RELEASE")`. But I think that's of very little use -- I would
personally grep for just "CFG_RELEASE" -- and it complicates the macro,
requiring the use of `syn`.

This commit simplifies the macro.

compiler/rustc_macros/src/lib.rs

@@ -26,6 +26,9 @@ mod symbols;
 mod type_foldable;
 mod type_visitable;
 
+// Reads the rust version (e.g. "1.75.0") from the CFG_RELEASE env var and
+// produces a `RustcVersion` literal containing that version (e.g.
+// `RustcVersion { major: 1, minor: 75, patch: 0 }`).
 #[proc_macro]
 pub fn current_rustc_version(input: TokenStream) -> TokenStream {
     current_version::current_version(input)