[clang] Fix CallExpr dependence bit may not respect all its arguments.

Before this patch, the dependence of CallExpr was only computed in the
constructor, the dependence bits might not reflect truth -- some arguments might
be not set (nullptr) during this time, e.g. CXXDefaultArgExpr will be set via
the setArg method in the later parsing stage, so we need to recompute the
dependence bits.