Tweak incorrect ABI suggestion

Provide a better suggestion message, and make the suggestion verbose.

```
error[E0703]: invalid ABI: found `riscv-interrupt`
  --> $DIR/riscv-discoverability-guidance.rs:17:8
   |
LL | extern "riscv-interrupt" fn isr() {}
   |        ^^^^^^^^^^^^^^^^^ invalid ABI
   |
   = note: invoke `rustc --print=calling-conventions` for a full list of supported calling conventions
help: there's a similarly named valid ABI `"riscv-interrupt-m"`
   |
LL | extern "riscv-interrupt-m" fn isr() {}
   |                        ++
```

compiler/rustc_ast_lowering/src/errors.rs

@@ -46,8 +46,9 @@ pub(crate) struct TupleStructWithDefault {
 #[derive(Subdiagnostic)]
 #[suggestion(
     ast_lowering_invalid_abi_suggestion,
-    code = "{suggestion}",
-    applicability = "maybe-incorrect"
+    code = "\"{suggestion}\"",
+    applicability = "maybe-incorrect",
+    style = "verbose"
 )]
 pub(crate) struct InvalidAbiSuggestion {
     #[primary_span]

compiler/rustc_ast_lowering/src/item.rs

@@ -1510,7 +1510,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             span: abi.span,
             suggestion: suggested_name.map(|suggested_name| InvalidAbiSuggestion {
                 span: abi.span,
-                suggestion: format!("\"{suggested_name}\""),
+                suggestion: suggested_name.to_string(),
             }),
             command: "rustc --print=calling-conventions".to_string(),
         });