Release notes for r158230 "Disallow using ObjC literals in direct comparisons"

Release notes for r158230 "Disallow using ObjC literals in direct comparisons"

This may turn out to be a controversial change, due to string literals being
uniqued at link time, but Apple's docs only say "The compiler makes such
object constants unique on a per-module basis..."[1] without actually saying
what a "module" is. (It's not a clang module.) Furthermore, this uniqueness
guarantee often can't be guaranteed once the string has been passed through
framework code.

If this does turn out very controversial, we could downgrade this to a
DefaultError warning for strings, and leave it as a true Error for other
kinds of literals.

(<rdar://problem/11300873>)

[1]: https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Strings/Articles/CreatingStrings.html

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159766 91177308-0d34-0410-b5e6-96231b3b80d8