Rollup merge of #87170 - xFrednet:clippy-5393-add-diagnostic-items, r=Manishearth,oli-obk

Add diagnostic items for Clippy

This adds a bunch of diagnostic items to `std`/`core`/`alloc` functions, structs and traits used in Clippy. The actual refactorings in Clippy to use these items will be done in a different PR in Clippy after the next sync.

This PR doesn't include all paths Clippy uses, I've only gone through the first 85 lines of Clippy's [`paths.rs`](https://github.com/rust-lang/rust-clippy/blob/ecf85f4bdc319f9d9d853d1fff68a8a25e64c7a8/clippy_utils/src/paths.rs) (after rust-lang/rust-clippy#7466) to get some feedback early on. I've also decided against adding diagnostic items to methods, as it would be nicer and more scalable to access them in a nicer fashion, like adding a `is_diagnostic_assoc_item(did, sym::Iterator, sym::map)` function or something similar (Suggested by `@camsteffen` [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/147480-t-compiler.2Fwg-diagnostics/topic/Diagnostic.20Item.20Naming.20Convention.3F/near/225024603))

There seems to be some different naming conventions when it comes to diagnostic items, some use UpperCamelCase (`BinaryHeap`) and some snake_case (`hashmap_type`). This PR uses UpperCamelCase for structs and traits and snake_case with the module name as a prefix for functions. Any feedback on is this welcome.

cc: rust-lang/rust-clippy#5393

r? `@Manishearth`

Author: JohnTitor

collections/btree/map/entry.rs

@@ -14,6 +14,7 @@ use Entry::*;
 ///
 /// [`entry`]: BTreeMap::entry
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "BTreeEntry")]
 pub enum Entry<'a, K: 'a, V: 'a> {
     /// A vacant entry.
     #[stable(feature = "rust1", since = "1.0.0")]