[flang][hlfir] Fix hlfir.set_length codegen

The bufferization pass was propagating the raw alloca storage
(which may not allow to later retrieve the length) instead of
the hlfir variable value (which is guaranteed to hold the
character length).

Fix this and makes packageBufferizedExpr "storage" argument and
getBufferizedExprStorage return an hlfir::Entity to avoid similar
error in the future (the caller of packageBufferizedExpr will have
to think a bit when adding the explicit hlfir::Entity{} cast).

Differential Revision: https://reviews.llvm.org/D148307