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_middle/src/traits/mod.rs

@@ -1010,10 +1010,7 @@ impl ObjectSafetyViolation {
             ) => {
                 err.span_suggestion(
                     *span,
-                    &format!(
-                        "consider changing method `{}`'s `self` parameter to be `&self`",
-                        name
-                    ),
+                    format!("consider changing method `{}`'s `self` parameter to be `&self`", name),
                     "&Self",
                     Applicability::MachineApplicable,
                 );
@@ -1021,7 +1018,7 @@ impl ObjectSafetyViolation {
             ObjectSafetyViolation::AssocConst(name, _)
             | ObjectSafetyViolation::GAT(name, _)
             | ObjectSafetyViolation::Method(name, ..) => {
-                err.help(&format!("consider moving `{}` to another trait", name));
+                err.help(format!("consider moving `{}` to another trait", name));
             }
         }
     }