[analyzer] Put back DefaultBool's implicit conversion to bool.

DefaultBool is basically just "bool with a default constructor", so it
really should implicitly convert to bool. In fact, it should convert to
bool&, so that it could be passed to functions that take bools by reference.

This time, mark the operator bool& as implicit to promise that it's
deliberate.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181908 91177308-0d34-0410-b5e6-96231b3b80d8