Reduce diagnostic type visibilities.

Most diagnostic types are only used within their own crate, and so have
a `pub(crate)` visibility. We have some diagnostic types that are
unnecessarily `pub`. This is bad because (a) information hiding, and (b)
if a `pub(crate)` type becomes unused the compiler will warn but it
won't warn for a `pub` type.

This commit eliminates unnecessary `pub` visibilities for some
diagnostic types, and also some related things due to knock-on effects.
(I found these types with some ad hoc use of `grep`.)

compiler/rustc_driver_impl/src/session_diagnostics.rs

@@ -4,7 +4,7 @@
 
 #[derive(Diagnostic)]
 #[diag("could not emit MIR: {$error}")]
-pub struct CantEmitMIR {
+pub(crate) struct CantEmitMIR {
     pub error: std::io::Error,
 }