[flang] Add High level Fortran IR dialect

This patch adds the basic dialect definition of the HLFIR dialect that
was described in https://reviews.llvm.org/D134285.

It adds the definition of the hlfir.expr type and related tests so that
it can be verified that the dialect is properly hooked up by the tools.

Operations will be added as progress is made in the expression
lowering update.

Differential Revision: https://reviews.llvm.org/D136328