[flang][NFC] rename hlfir::FortranEntity into EntityWithAttributes

This reflects the fact that Attributes will not always be visible when
looking at an HLFIR variable. The EntityWithAttributes class is used
to denote in the compiler code that the value at hand has visible
attributes. It is intended to be used in lowering so that the code
can query about operands attributes when generating code.

Differential Revision: https://reviews.llvm.org/D137792