[mlir] Refactor how parser/printers are specified for AttrDef/TypeDef

There is currently an awkwardly complex set of rules for how a
parser/printer is generated for AttrDef/TypeDef. It can change depending on if a
mnemonic was specified, if there are parameters, if using the assemblyFormat, if
individual parser/printer code blocks were specified, etc. This commit refactors
this to make what the attribute/type wants more explicit, and to better align
with how formats are specified for operations.

Firstly, the parser/printer code blocks are removed in favor of a
`hasCustomAssemblyFormat` bit field. This aligns with the operation format
specification (and is nice to remove code blocks from ODS).

This commit also adds a requirement to explicitly set `assemblyFormat` or
`hasCustomAssemblyFormat` when the mnemonic is set and the attr/type
has no parameters. This removes the weird implicit matrix of behavior,
and also encourages the author to make a conscious choice of either C++
or declarative format instead of implicitly opting them into the C++
format (we should be pushing towards declarative when possible).

Differential Revision: https://reviews.llvm.org/D121505

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