librustc: Parse, but do not fully turn on, the `ref` keyword for
by-reference upvars.

This partially implements RFC 38. A snapshot will be needed to turn this
on, because stage0 cannot yet parse the keyword.

Part of #12381.
auto merge of #15929 : pcwalton/rust/by-ref-closures, r=alexcrichton

by-reference upvars.

This partially implements RFC 38. A snapshot will be needed to turn this
on, because stage0 cannot yet parse the keyword.

Part of #12831.

r? @alexcrichton