[mlir] Add macro for enabling all generated pass declarations

Currently the generated pass declarations have to be enabled per-pass
using multiple `GEN_PASS_DECL_{PASSNAME}` defines. This adds
`GEN_PASS_DECL`, which enables the declarations for all passes in the
group with a single macro. This is convenient for cases where a single
header is used for all passes in the group.

Reviewed By: mehdi_amini, mscuttari

Differential Revision: https://reviews.llvm.org/D134766