[flang][hlfir] Add assignment mask operations

Add hlfir.forall_mask, hlfir.where, and hlfir.elsewhere operations that
are operations that holds (optionally for hlfir.elsewhere) the
evaluation of a logical mask that controls the evaluation of nested
operations.

They allow representing Fortran forall control mask, as well as where
and eslewhere statements/constructs.

They use the OrderedAssignmentTreeOpInterface since they can all be used
inside Forall and their masks should be fully evaluated for all the
index-value set induced by parent Forall before any of the nested
operations in their body is evaluated.

I initially tried making them into a single operation with some attributes
to make a difference, but I felt this made the verifier/parser/printer and
usages messier/tricky compared to making three distinct operations that
represent the three Fortran feature in a vanilla way.

Differential Revision: https://reviews.llvm.org/D149754