Endian fix for Strings in s390x

[garyliu1]

What's in this pull request?

This PR contains two big endian related fix in String.

1. The change in StaticString's withUTF8Buffer is to store the UTF-8 code unit properly in the buffer.
2. The change in StringCore's _nthContiguous is to represent the UTF-16 data correctly when it contains the high byte.

We ran validation tests and foundation tests with the change on x86-64 and s390x. No new regression is observed.

Resolved bug number: (SR-)

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[jrose-apple]

Are these changes caught by existing tests, or is there a new test that should be added so that this would fail on big-endian systems without this change?

[garyliu1]

@jrose-apple, thanks for the comment. There are existing tests catching these issues in s390x which I used for debugging.

1_stdlib/StaticSting.swift is catching UTF-8 one and 1_stdlib/Character.swift is catching the UTF-16 issue

Current test coverage would be sufficient.

[garyliu1]

@swift-ci Please smoke test OS X platform

[CodaFi]

@garyliu1 Unfortunately, that won't work without privileges!

@swift-ci please smoke test OS X platform.

[garyliu1]

@CodaFi. Thank you for getting smoke test running

[jrose-apple]

Thanks, Gary! (and Robert). Given @vivkong's comment on the other s390x patch, it probably makes sense to pull this into swift-3.0-branch (following the release process). Mind making a pull request for that?

[garyliu1]

@jrose-apple Thanks. We will follow up with another PR to swift-3.0-branch.