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/diagnostics.rs

@@ -104,7 +104,7 @@ impl MachineStopType for TerminationInfo {
         self: Box<Self>,
         _: &mut dyn FnMut(
             std::borrow::Cow<'static, str>,
-            rustc_errors::DiagnosticArgValue<'static>,
+            rustc_errors::DiagnosticArgValue,
         ),
     ) {
     }