Remove the lifetime from DiagnosticArgValue.

Because it's almost always static.

This makes `impl IntoDiagnosticArg for DiagnosticArgValue` trivial,
which is nice.

There are a few diagnostics constructed in
`compiler/rustc_mir_build/src/check_unsafety.rs` and
`compiler/rustc_mir_transform/src/errors.rs` that now need symbols
converted to `String` with `to_string` instead of `&str` with `as_str`,
but that' no big deal, and worth it for the simplifications elsewhere.

Author: nnethercote

src/errors.rs

@@ -35,7 +35,7 @@ pub(crate) enum PossibleFeature<'a> {
 struct ExitCode(Option<i32>);
 
 impl IntoDiagnosticArg for ExitCode {
-    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue {
         let ExitCode(exit_code) = self;
         match exit_code {
             Some(t) => t.into_diagnostic_arg(),