[MLIR] Split autogenerated pass declarations & C++ controllable pass options

The pass tablegen backend has been reworked to remove the monolithic nature of the autogenerated declarations.
The pass public header can be generated with the -gen-pass-decls option. It contains options structs and registrations: the inclusion of options structs can be controlled individually for each pass by defining the GEN_PASS_DECL_PASSNAME macro; the declaration of the registrations have been kept together and can still be included by defining the GEN_PASS_REGISTRATION macro.
The private code used for the pass implementation (i.e. the pass base class and the constructors definitions, if missing from tablegen) can be generated with the -gen-pass-defs option. Similarly to the declarations file, the definitions of each pass can be enabled by defining the GEN_PASS_DEF_PASNAME variable.
While doing so, the pass base class has been enriched to also accept a the aformentioned struct of options and copy them to the actual pass options, thus allowing each pass to also be configurable within C++ and not only through command line.

Reviewed By: rriddle, mehdi_amini, Mogball, jpienaar

Differential Revision: https://reviews.llvm.org/D131839