[flang][openacc] Make sure the correct flags are set for symbol in acc declare

Flags were not correctly set for symbols appearing in the OpenACC declare
directive in module declaration part.

Also some missing flags for OpenACC are added. This makes the Flags enum
> 64 and then the implementation switch to std::bitset as defined in
`flang/include/flang/Common/enum-set.h`. Therefore, `constexpr` cannot be
used for set of flags in `flang/lib/Semantics/resolve-directives.cpp`.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D157042