swift234

[irgen] Lower fix_lifetime => swift_keepAlive().

I introduced a function swift_keepAlive2() which has a different signature from
swift_keepAlive() until I can verify that the stdlib is using the new
infrastructure.

The difference in signature is that swift_keepAlive2 takes just a pointer while
swift_keepAlive also takes a metadata value that is not necessary for our
purposes anymore.

Swift SVN r21718