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`).

src/pretty_clif.rs

@@ -227,7 +227,7 @@ pub(crate) fn write_ir_file(
         // Using early_warn as no Session is available here
         rustc_session::early_warn(
             rustc_session::config::ErrorOutputType::default(),
-            &format!("error writing ir file: {}", err),
+            format!("error writing ir file: {}", err),
         );
     }
 }