Change SWIFT_OPTIMIZATION_LEVEL to fast, single file
Change SWIFT_OPTIMIZATION_LEVEL to fast, single file

This is a workaroud for #2109.

couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios,couchbase/couchbase-lite-ios