[flang] Catch bad usage of POINTER attribute

Most attributes apply to only object or only procedure entities,
and attempts to apply them to other kinds of symbol table entries
are caught in name resolution when ConvertToObjectEntity() or
ConvertToProcEntity() fails.  However, the POINTER attribute can
be applied to both, and name resolution can't perform that conversion
yet, and as a result we don't catch many kinds of silly errors.
Fix by ensuring that the symbol is of a type that could eventually
become an object or procedure entity if it is not one already.

Differential Revision: https://reviews.llvm.org/D140137