Auto merge of #4508 - rust-lang:reduced-symbolism, r=phansch
Auto merge of #4508 - rust-lang:reduced-symbolism, r=phansch

Simplify `utils::match_def_path`, removing a FIXME

changelog: none

This removes the `Vec<Symbol>` allocation. We still need to call `cx.get_def_path`, but this should already have been interned, and I don't see how we can keep ergonomics of that function without allocating a `Vec`.

r? @phansch

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust