"Enhance" CheckArithmeticConstantExpression to accept ?: with a constant

"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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57153 91177308-0d34-0410-b5e6-96231b3b80d8