"Enhance" CheckArithmeticConstantExpression to accept ?: with a constant

condition as a constant even if the unevaluated side is a not a constant.
We don't do this when extensions are off, and we emit a warning when this 
happens:

t.c:22:11: warning: expression is not a constant, but is accepted as one by GNU extensions
short t = __builtin_constant_p(5353) ? 42 : somefunc();
          ^                                 ~~~~~~~~~~

suggestions for improvement are welcome.  This is obviously horrible, but
is required for real-world code.

llvm-svn: 57153