[flang][hlfir] Fix multiple return declaration type

When the ENTRY statement is used, the same source can return different
types depending on the entry point. These different return values are
storage associated (share the same storage). Previously, this led to the
declaration of the results to all have the largest type. This patch adds
a convert between the stack allocation and the declaration so that the
hlfir.decl gets the right type.

I haven't managed to generate code where this convert converted a
reference to an allocation for a smaller type into an allocation for a
larger one, but I have added an assert just in case.

This is a different solution to https://reviews.llvm.org/D152725, see
discussion there.

Differential Revision: https://reviews.llvm.org/D152931