Compute the hash of all interface tokens when parsing; write the
interface hash to the swiftdeps file, or if the -dump-interface-hash
option is passed to the frontend. This hash will be used in incremental
mode to determine whether a file's interface has changed, and therefore
whether dependent files need to be rebuilt in response to the change.
Committed on ChrisW's behalf while he gets his setup unborked.
rdar://problem/15352929
Swift SVN r30477
