Don't crash on boxed strings when +stringWithUTF8String: is missing.

Also, unify some diagnostics for boxed expressions that have the same form.

Fixes PR12804.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156713 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed