Use Cow in {D,Subd}iagnosticMessage.

Each of `{D,Subd}iagnosticMessage::{Str,Eager}` has a comment:
```
// FIXME(davidtwco): can a `Cow<'static, str>` be used here?
```
This commit answers that question in the affirmative. It's not the most
compelling change ever, but it might be worth merging.

This requires changing the `impl<'a> From<&'a str>` impls to `impl
From<&'static str>`, which involves a bunch of knock-on changes that
require/result in call sites being a little more precise about exactly
what kind of string they use to create errors, and not just `&str`. This
will result in fewer unnecessary allocations, though this will not have
any notable perf effects given that these are error paths.

Note that I was lazy within Clippy, using `to_string` in a few places to
preserve the existing string imprecision. I could have used `impl
Into<{D,Subd}iagnosticMessage>` in various places as is done in the
compiler, but that would have required changes to *many* call sites
(mostly changing `&format("...")` to `format!("...")`) which didn't seem
worthwhile.

compiler/rustc_session/src/parse.rs

@@ -123,7 +123,7 @@ pub fn feature_err_issue(
 /// Construct a future incompatibility diagnostic for a feature gate.
 ///
 /// This diagnostic is only a warning and *does not cause compilation to fail*.
-pub fn feature_warn(sess: &ParseSess, feature: Symbol, span: Span, explain: &str) {
+pub fn feature_warn(sess: &ParseSess, feature: Symbol, span: Span, explain: &'static str) {
     feature_warn_issue(sess, feature, span, GateIssue::Language, explain);
 }
 
@@ -140,7 +140,7 @@ pub fn feature_warn_issue(
     feature: Symbol,
     span: Span,
     issue: GateIssue,
-    explain: &str,
+    explain: &'static str,
 ) {
     let mut err = sess.span_diagnostic.struct_span_warn(span, explain);
     add_feature_diagnostics_for_issue(&mut err, sess, feature, issue);