[flang] Add ClassIs runtime function

Add a `ClassIs` function that takes a descriptor and a
type desc to implement the check needed by the CLASS IS type guard
in SELECT TYPE construct.
Since the kind type parameter are directly folded in the type itself
in Flang and the type descriptor is a global, the function just check
if the type descriptor address of the descriptor is equivalent to
the type descriptor address of the global. If not, it check in the
parents of the descriptor's type descriptor.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D138279