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_expand/src/base.rs

@@ -1154,7 +1154,7 @@ impl<'a> ExtCtxt<'a> {
         // Fixme: does this result in errors?
         self.expansions.clear();
     }
-    pub fn bug(&self, msg: &str) -> ! {
+    pub fn bug(&self, msg: &'static str) -> ! {
         self.sess.parse_sess.span_diagnostic.bug(msg);
     }
     pub fn trace_macros(&self) -> bool {
@@ -1224,7 +1224,7 @@ pub fn resolve_path(
 pub fn expr_to_spanned_string<'a>(
     cx: &'a mut ExtCtxt<'_>,
     expr: P<ast::Expr>,
-    err_msg: &str,
+    err_msg: &'static str,
 ) -> Result<(Symbol, ast::StrStyle, Span), Option<(DiagnosticBuilder<'a, ErrorGuaranteed>, bool)>> {
     // Perform eager expansion on the expression.
     // We want to be able to handle e.g., `concat!("foo", "bar")`.
@@ -1262,7 +1262,7 @@ pub fn expr_to_spanned_string<'a>(
 pub fn expr_to_string(
     cx: &mut ExtCtxt<'_>,
     expr: P<ast::Expr>,
-    err_msg: &str,
+    err_msg: &'static str,
 ) -> Option<(Symbol, ast::StrStyle)> {
     expr_to_spanned_string(cx, expr, err_msg)
         .map_err(|err| {