A burst of progress on the GCC Rust front end

Arthur Cohen has posted a massive series of patches in four parts(part1,part2,part3,part4)upstreaming all of the recent work on the GCC Rust front end. Thesechanges include the Polonius borrow checker, the foreign-functioninterface, inline assembly support, if-let statement handling,multiple built-in derive macros, for loops, and more.