macros: add args for non-subdiagnostic fields

Non-subdiagnostic fields (i.e. those that don't have `#[label]`
attributes or similar and are just additional context) have to be added
as arguments for Fluent messages to refer them. This commit extends the
`SessionDiagnostic` derive to do this for all fields that do not have
attributes and introduces an `IntoDiagnosticArg` trait that is
implemented on all types that can be converted to a argument for Fluent.

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

compiler/rustc_errors/src/lib.rs

@@ -406,7 +406,8 @@ impl fmt::Display for ExplicitBug {
 impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{
-    Diagnostic, DiagnosticArg, DiagnosticId, DiagnosticStyledString, SubDiagnostic,
+    Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId, DiagnosticStyledString,
+    IntoDiagnosticArg, SubDiagnostic,
 };
 pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
 use std::backtrace::Backtrace;