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.

compiler/rustc_session/src/session.rs

@@ -461,7 +461,7 @@ impl Session {
     }
     #[rustc_lint_diagnostics]
     pub fn fatal(&self, msg: impl Into<DiagnosticMessage>) -> ! {
-        self.diagnostic().fatal(msg).raise()
+        self.diagnostic().fatal(msg)
     }
     #[rustc_lint_diagnostics]
     #[track_caller]