Change the type of ObjCStringLiteral from "struct __builtin_CFString *" to "NSConstantString *".

This makes the typecheck much happier. Without this change, the type checker would have to special case "struct __builtin_CFString *". This change does assume the interface for NSConstantString is declared in the translation unit.

I left ASTContext::getCFConstantStringType() around for now (with a comment that says it is currently unused).

llvm-svn: 43021