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_builtin_macros/src/source_util.rs

@@ -150,7 +150,7 @@ pub fn expand_include<'cx>(
                         if self.p.token != token::Eof {
                             let token = pprust::token_to_string(&self.p.token);
                             let msg = format!("expected item, found `{}`", token);
-                            self.p.struct_span_err(self.p.token.span, &msg).emit();
+                            self.p.struct_span_err(self.p.token.span, msg).emit();
                         }
 
                         break;