NSString addingPercentEncoding methods should return nil for invalid UTF-16

[jrflat]

The new Swift implementation for -[NSString addingPercentEncoding...] methods should return nil for NSStrings with invalid (unpaired) UTF-16 surrogates instead of using the U+FFFD replacement character.

Motivation:

Restore previous behavior of returning nil when an NSString contains an unpaired UTF-16 surrogate.

Modifications:

The extensions now validate UTF-16 backed NSStrings via String(validating: ..., as: UTF16.self) instead of just bridging to String (which replaces invalid UTF-16 with U+FFFD when accessing the bytes via .withUTF8 { ... }).

Result:

No behavior change for swift-foundation. On Darwin, these NSString methods maintain their old behavior when the Swift implementation is enabled.

Testing:

NSString testing of these methods for strings containing unpaired UTF-16 surrogates.

[jrflat]

@swift-ci please test