Fixed bug in CFG construction where we did not properly handle the GCC

Fixed bug in CFG construction where we did not properly handle the GCC
extension "?:" for the ternary operator, e.g.: x ?: y; This expression is
represented in the clang ASTs as a ConditionalOperator whose LHS expression is
NULL. Now we handle this special case, causing the block containing the
condition to be a predecessor to the block that "merges" the values of the
ternary operator.

Thanks to Nuno Lopes for identifying and diagnosing this bug!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44327 91177308-0d34-0410-b5e6-96231b3b80d8