[mlir][ods] Fix attribute type handling in op assembly formats

[Mogball]

There are some oversights and missing features when it comes to handling attribute types in operation assembly formats.

To list a few:

1. Specifying type elision (type-elision directive?)
2. Parsing attribute types with parseCustomAttributeWithFallback is not possible because the parse doesn't look ahead

And a few other things @River707

[llvmbot]

@llvm/issue-subscribers-mlir-core