`cfg` completions are duplicated many times

[jonas-schievink]

I'm assuming this is because cfg flags may be set with mulitple values, eg. target_has_atomic = "ptr" and target_has_atomic = "8" may both be set at once. However, our completions don't actually complete that part of the directive. Perhaps they should, or if not (since it's reasonable to write cfgs that are inactive) they should be deduplicated.

[Veykril]

Relevant code is here

rust-analyzer/crates/ide-completion/src/completions/attribute/cfg.rs

Lines 37 to 40 in 6fc5c3c

	None => ctx.krate.potential_cfg(ctx.db).get_cfg_keys().cloned().for_each(|s| {
	let item = CompletionItem::new(SymbolKind::BuiltinAttr, ctx.source_range(), s);
	acc.add(item.build());
	}),