Rollup merge of #160757 - JonathanBrouwer:attr_parsing_diagnostics, r=GuillaumeGomez

Merge `rustc_attr_parsing/session_diagnostics.rs` into `diagnostics.rs`

Renaming diagnostics files to all be diagnostics.rs, per the decision in https://github.com/rust-lang/compiler-team/issues/1003

r? @GuillaumeGomez