stdlib: introduce UnicodeScalarLiteralConvertible protocol
stdlib: introduce UnicodeScalarLiteralConvertible protocol

This allows UnicodeScalars to be constructed from an integer, rather
then from a string.  Not only this avoids an unnecessary memory
allocation (!) when creating a UnicodeScalar, this also allows the
compiler to statically check that the string contains a single scalar
value (in the same way the compiler checks that Character contains only
a single extended grapheme cluster).

rdar://17966622

Swift SVN r21198

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation