Restrict From<S> for {D,Subd}iagnosticMessage.

Currently a `{D,Subd}iagnosticMessage` can be created from any type that
impls `Into<String>`. That includes `&str`, `String`, and `Cow<'static,
str>`, which are reasonable. It also includes `&String`, which is pretty
weird, and results in many places making unnecessary allocations for
patterns like this:
```
self.fatal(&format!(...))
```
This creates a string with `format!`, takes a reference, passes the
reference to `fatal`, which does an `into()`, which clones the
reference, doing a second allocation. Two allocations for a single
string, bleh.

This commit changes the `From` impls so that you can only create a
`{D,Subd}iagnosticMessage` from `&str`, `String`, or `Cow<'static,
str>`. This requires changing all the places that currently create one
from a `&String`. Most of these are of the `&format!(...)` form
described above; each one removes an unnecessary static `&`, plus an
allocation when executed. There are also a few places where the existing
use of `&String` was more reasonable; these now just use `clone()` at
the call site.

As well as making the code nicer and more efficient, this is a step
towards possibly using `Cow<'static, str>` in
`{D,Subd}iagnosticMessage::{Str,Eager}`. That would require changing
the `From<&'a str>` impls to `From<&'static str>`, which is doable, but
I'm not yet sure if it's worthwhile.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -67,7 +67,7 @@ impl IntercrateAmbiguityCause {
     /// Emits notes when the overlap is caused by complex intercrate ambiguities.
     /// See #23980 for details.
     pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
-        err.note(&self.intercrate_ambiguity_hint());
+        err.note(self.intercrate_ambiguity_hint());
     }
 
     pub fn intercrate_ambiguity_hint(&self) -> String {
@@ -2449,7 +2449,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 // for a variable being generalized...
                 let guar = self.infcx.tcx.sess.delay_span_bug(
                     obligation.cause.span,
-                    &format!(
+                    format!(
                         "Impl {:?} was matchable against {:?} but now is not",
                         impl_def_id, obligation
                     ),