[clang][Interp] Optionally cast comparison result to non-bool

Our comparison opcodes always produce a Boolean value and push it on the
stack. However, the result of such a comparison in C is int, so the
later code expects an integer value on the stack.

Work around this problem by casting the boolean value to int in those
cases. This is not ideal for C however. The comparison is usually
wrapped in a IntegerToBool cast anyway.

Differential Revision: https://reviews.llvm.org/D149645