Auto merge of #61140 - estebank:attr-diagnostics, r=michaelwoerister

Auto merge of #61140 - estebank:attr-diagnostics, r=michaelwoerister

Reword malformed attribute input diagnostics

- Handle empty `cfg_attr` attribute
- Reword empty `derive` attribute error
- Use consistend error message: "malformed `attrname` attribute input"
- Provide suggestions when possible
- Move note/help to label/suggestion
- Use consistent wording "ill-formed" -> "malformed"
- Move diagnostic logic out of parser

Split up from https://github.com/rust-lang/rust/pull/61026, where there's prior conversation.