[se-0405] Implement API additions

[glessard]

API additions (and tests) from SE-0405, namely:

String.init?<Encoding>(
    validating codeUnits: some Sequence<Encoding.CodeUnit>,
    as encoding: Encoding.Type
  ) where Encoding: Unicode.Encoding

String.init?<Encoding>(
    validating codeUnits: some Sequence<Int8>,
    as encoding: Encoding.Type
  ) where Encoding: Unicode.Encoding, Encoding.CodeUnit == UInt8

The API renaming will come in a separate PR.

rdar://114999766

[glessard]

@swift-ci please test

[glessard]

GitHub is failing to pick up a push to this branch. Weird.

[glessard]

Re-opened as #68419

[simanerush]

@glessard I had this problem yesterday with my branch and I solved it but pushing an empty commit and then deleting it