[ODS/AsmParser] Don't pass MLIRContext with DialectAsmParser.

The former is redundant because the later carries it as part of
its builder.  Add a getContext() helper method to DialectAsmParser
to make this more convenient, and stop passing the context around
explicitly.  This simplifies ODS generated parser hooks for attrs
and types.

This resolves PR51985

Differential Revision: https://reviews.llvm.org/D110796