[stdlib] Land UnicodeEncoding
Several tests needed to be adjusted:
* test/SourceKit/CodeComplete/complete_moduleimportdepth.swift is fragile and
the order of some test results needed to be rearranged due to nondeterministic
effects of some kind (checked with Argyrios).
* test/stdlib/Renames.swift was checking obsolete migration messages
* test/stdlib/TestCharacterSet.swift seems(?) to have exposed a bug in
Foundation or its overlay; reported in <rdar://problem/31706753>
