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.

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -34,7 +34,7 @@ impl MachineStopType for ConstEvalErrKind {
     }
     fn add_args(
         self: Box<Self>,
-        adder: &mut dyn FnMut(std::borrow::Cow<'static, str>, DiagnosticArgValue<'static>),
+        adder: &mut dyn FnMut(std::borrow::Cow<'static, str>, DiagnosticArgValue),
     ) {
         use ConstEvalErrKind::*;
         match *self {