Fix legacy diagnostic for Display-like macros

[tyranron]

Related to #225

Synopsis

Diagnostic introduced in #225 regarding old syntax usage, displays incorrect message in case formatting arguments are specified as identifiers (not string literals):

#[derive(derive_more::Display)]
#[display(fmt = "Stuff({}): {}", _0)]
pub struct Bar(String);

error: unknown attribute, expected `bound(...)`
 --> tests/compile_fail/display/legacy_fmt_syntax.rs:8:11
  |
8 | #[display(fmt = "Stuff({}): {}", _0)]
  |           ^^^

In my experience, this situation happens very often in the wild.

Solution

Fix the diagnostic to consider ident formatting arguments properly.

Checklist

• Documentation is updated (not required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (not required)