Rollup merge of #161258 - xmakro:perf/in-external-macro-root-fast-path, r=petrochenkov

perf: return early from in_external_macro for root contexts

This line was hot when profiling clippy-driver. With this change, clippy instructions reduce for ryu -1.01%, syn -0.21%, regex -0.20%.

r? @petrochenkov