[OpenMP] Make the filetered clause iterator a real iterator and type safe.

This replaces the filtered generic iterator with a type-specfic one based
on dyn_cast instead of comparing the kind enum. This allows us to use
range-based for loops and eliminates casts. No functionality change
intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246384 91177308-0d34-0410-b5e6-96231b3b80d8