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!

llvm-svn: 44327