errors: introduce DiagnosticMessage

Introduce a `DiagnosticMessage` type that will enable diagnostic
messages to be simple strings or Fluent identifiers.
`DiagnosticMessage` is now used in the implementation of the standard
`DiagnosticBuilder` APIs.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_errors/src/lib.rs

@@ -145,7 +145,7 @@ pub struct CodeSuggestion {
     /// ]
     /// ```
     pub substitutions: Vec<Substitution>,
-    pub msg: String,
+    pub msg: DiagnosticMessage,
     /// Visual representation of this suggestion.
     pub style: SuggestionStyle,
     /// Whether or not the suggestion is approximate
@@ -400,7 +400,9 @@ impl fmt::Display for ExplicitBug {
 
 impl error::Error for ExplicitBug {}
 
-pub use diagnostic::{Diagnostic, DiagnosticId, DiagnosticStyledString, SubDiagnostic};
+pub use diagnostic::{
+    Diagnostic, DiagnosticId, DiagnosticMessage, DiagnosticStyledString, SubDiagnostic,
+};
 pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
 use std::backtrace::Backtrace;