[-Wunreachable-code] Handle Objective-C bool literals in 'isConfigurationValue'.

This includes special casing 'YES' and 'NO', which are constants
defined as macros.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@203380 91177308-0d34-0410-b5e6-96231b3b80d8