Implement the final (hopefully) wrinkle to i-c-e + builtin_constant_p

processing: it allows arbitrary foldable constants as the operand of ?: when
builtin_constant_p is the condition.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60954 91177308-0d34-0410-b5e6-96231b3b80d8