When producing frontend arguments for sourcekitd, force the output mode
to -typecheck so that we do not create any temporary output files in the
driver. Previously, any sourcekitd operation that created a compiler
invocation would create 0-sized .o file inside $TMPDIR that would never
be cleaned up.
The new swift-driver project handles temporaries much better as
VirtualPath, and should not need this approach.
rdar://62366123
