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_metadata/src/creader.rs

@@ -943,7 +943,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
                     lint::builtin::UNUSED_CRATE_DEPENDENCIES,
                     span,
                     ast::CRATE_NODE_ID,
-                    &format!(
+                    format!(
                         "external crate `{}` unused in `{}`: remove the dependency or add `use {} as _;`",
                         name,
                         self.tcx.crate_name(LOCAL_CRATE),