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_interface/src/interface.rs

@@ -80,7 +80,7 @@ pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String
                     ($reason: expr) => {
                         early_error(
                             ErrorOutputType::default(),
-                            &format!(concat!("invalid `--cfg` argument: `{}` (", $reason, ")"), s),
+                            format!(concat!("invalid `--cfg` argument: `{}` (", $reason, ")"), s),
                         );
                     };
                 }
@@ -139,10 +139,7 @@ pub fn parse_check_cfg(specs: Vec<String>) -> CheckCfg {
                 ($reason: expr) => {
                     early_error(
                         ErrorOutputType::default(),
-                        &format!(
-                            concat!("invalid `--check-cfg` argument: `{}` (", $reason, ")"),
-                            s
-                        ),
+                        format!(concat!("invalid `--check-cfg` argument: `{}` (", $reason, ")"), s),
                     )
                 };
             }