The Swift compiler uses files with an extension of ".swiftdeps" to store
information about cross-file dependencies. These files are read in at the
start of compilation to compute a dependency graph, and updated as compilation
proceeds. However, because these files are updated on every build, an
issue with dependency analysis is hard to reproduce—the inputs have been
lost.
Address this by renaming swiftdeps files that are about to be
overwritten, to '.swiftdeps~'. This preserves dependency information
from the most recent compilation (but no further back).
