Avoid &format("...") calls in error message code.

Error message all end up passing into a function as an `impl
Into<{D,Subd}iagnosticMessage>`. If an error message is creatd as
`&format("...")` that means we allocate a string (in the `format!`
call), then take a reference, and then clone (allocating again) the
reference to produce the `{D,Subd}iagnosticMessage`, which is silly.

This commit removes the leading `&` from a lot of these cases. This
means the original `String` is moved into the
`{D,Subd}iagnosticMessage`, avoiding the double allocations. This
requires changing some function argument types from `&str` to `String`
(when all arguments are `String`) or `impl
Into<{D,Subd}iagnosticMessage>` (when some arguments are `String` and
some are `&str`).

compiler/rustc_parse/src/lexer/mod.rs

@@ -662,7 +662,7 @@ impl<'a> StringReader<'a> {
                 &RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
                 prefix_span,
                 ast::CRATE_NODE_ID,
-                &format!("prefix `{prefix}` is unknown"),
+                format!("prefix `{prefix}` is unknown"),
                 BuiltinLintDiagnostics::ReservedPrefix(prefix_span),
             );
         }