Ensure that we use the correct python to run the python based tools.
This also allows these tools to run on Windows which will not
necessarily associate the python script with an interpreter (python).
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
