[flang] add hlfir::FortranEntity class

Add hlfir::FortranEntity class and a first helper to convert it to
fir::ExtendedValue.

The hlfir::FortranEntity will be the core class of the new expression
lowering. It is conceptually very similar to what fir::ExtendedValue
is today, except that it is wrapping single mlir::Value: it holds the
SSA value for a lowered Fortran variable or expression value.

Differential Revision: https://reviews.llvm.org/D136428