Objective-C. Make diagnostics and fix-its consistent 
when diagnosing casting of a cstring literal to
NSString in default and -fobjc-arc mode. 
// rdar://14106083


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197515 91177308-0d34-0410-b5e6-96231b3b80d8