swift234

If the generated header doesn't change, don't touch the original file.

Mixed-source targets generally include the generated <Product>-Swift.h
header in many Objective-C (.m) source files. Any change to a Swift file
results in the header being touched, which means all the .m files get
recompiled.

This change makes Swift write the generated header to a temporary file
first, then use the previous commit's moveFileIfDifferent to put it in
its final destination.

rdar://problem/20553459

Swift SVN r28042