Auto merge of #23359 - erickt:quote, r=pnkfelix

This PR allows the quote macros to unquote trait items, impl items, where clauses, and paths.

diff --cc src/libsyntax/ext/quote.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge