[flang] Add kernel to lower expressions to HLFIR

This patch adds the kernel to lower evaluate::Expr to HLFIR to a
hlfir::FortranEntity (a single mlir::Value that can be interpreted as
a Fortran variable or the value of a Fortram expression).

It implements lowering of simple name designators ("x") and starts
adding a translation layer in AbstractConverter::genExprBox and
AbstractConverter::genExprAddr so that the new expression lowering
can be used without any changes for now in the current statement and
construct lowering.

Differential Revision: https://reviews.llvm.org/D136453