Avoid need for {D,Subd}iagnosticMessage imports.

The `fluent_messages!` macro produces uses of
`crate::{D,Subd}iagnosticMessage`, which means that every crate using
the macro must have this import:
```
use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
```

This commit changes the macro to instead use
`rustc_errors::{D,Subd}iagnosticMessage`, which avoids the need for the
imports.

compiler/rustc_mir_dataflow/src/lib.rs

@@ -13,7 +13,6 @@ extern crate tracing;
 #[macro_use]
 extern crate rustc_middle;
 
-use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
 use rustc_fluent_macro::fluent_messages;
 use rustc_middle::ty;