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
