[flang][RFC] Do not rely on attributes to tag HLFIR variable uses

After more considerations and experience, switch to one of the
alternative plan for HLFIR variable that will avoid requiring naming
designators and having to maintain and update names in attributes after
inlining of code duplication.

The cost is the increase of fir.box usage, which in most cases should
be removed when lowering from HLFIR to FIR.

Differential Revision: https://reviews.llvm.org/D137634