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_session/src/version.rs

@@ -9,7 +9,7 @@ pub struct RustcVersion {
 }
 
 impl RustcVersion {
-    pub const CURRENT: Self = current_rustc_version!(env!("CFG_RELEASE"));
+    pub const CURRENT: Self = current_rustc_version!();
 }
 
 impl Display for RustcVersion {