[perf] rustdoc: Perform less work when cleaning middle::ty parenthesized generic args CC #132697. I presume the perf regression it caused (if real) boils down to query invocation overhead, namely of `def_kind` & `trait_def` as we don't seem to be decoding more often from the crate metadata. I won't try the obvious and reduce the amount of query calls by threading information via params as that would render the code awkward. So instead I'm simply trying to attack some low-hanging fruits in the vicinity. --- Previously, we would `clean_middle_generic_args` *unconditionally* inside `clean_middle_generic_args_with_constraints` even though we didn't actually use its result for parenthesized generic args (`Trait(...) -> ...`). Now, we only call `clean_middle_generic_args` when necessary. Lastly, I've simplified `clean_middle_generic_args_with_constraints`. --- r? ghost