[flang][RFC] Adding higher level FIR ops to ease expression lowering

This document describes a new HLFIR dialect with a new value type
(hlfir.expr) and some new higher level FIR operations to make lowering
to FIR more straightforward and make pattern matching of high level
Fortran concepts (array and character assignments, character operations,
and transformational intrinsics) easier in FIR.

This should allow implementing the remaining gaps in Fortran 95 features
without increasing the lowering code complexity too much, and get a
clean start for the remaining F2003 and F2018 features.

Differential Revision: https://reviews.llvm.org/D134285