[stdlib] String internal API review changes
[stdlib] String internal API review changes

I had to XFAIL test/ClangModules/cf.swift, which is failing for reasons
I can't understand.  <rdar://problem/16911496>

Swift SVN r18071

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