Give Handler::fatal and Session::fatal the same return type.

Currently, `Handler::fatal` returns `FatalError`. But `Session::fatal`
returns `!`, because it calls `Handler::fatal` and then calls `raise` on
the result. This inconsistency is unfortunate.

This commit changes `Handler::fatal` to do the `raise` itself, changing
its return type to `!`. This is safe because there are only two calls to
`Handler::fatal`, one in `rustc_session` and one in
`rustc_codegen_cranelift`, and they both call `raise` on the result.

`HandlerInner::fatal` still returns `FatalError`, so I renamed it
`fatal_no_raise` to emphasise the return type difference.

src/concurrency_limiter.rs

@@ -64,7 +64,7 @@ impl ConcurrencyLimiter {
                     // Make sure to drop the mutex guard first to prevent poisoning the mutex.
                     drop(state);
                     if let Some(err) = err {
-                        handler.fatal(err).raise();
+                        handler.fatal(err);
                     } else {
                         // The error was already emitted, but compilation continued. Raise a silent
                         // fatal error.