swift234

[stdlib] Fix the `String.decodeCString` for UTF16 and UTF32 (#2681)

[stdlib] Fix the `String.decodeCString` for UTF16 and UTF32

Resolves [SR-1578](https://bugs.swift.org/browse/SR-1578]
Essentially the problem was that `strlen` is not the right way of
obtaining a length of anything but null-terminated UTF-8 sequence of
characters. Other encodings require alternative mechanisms.