[MLIR] Attribute and type formats in ODS

Declarative attribute and type formats with assembly formats. Define an
`assemblyFormat` field in attribute and type defs with a `mnemonic` to
generate a parser and printer.

```tablegen
def MyAttr : AttrDef<MyDialect, "MyAttr"> {
  let parameters = (ins "int64_t":$count, "AffineMap":$map);
  let mnemonic = "my_attr";
  let assemblyFormat = "`<` $count `,` $map `>`";
}
```

Use `struct` to define a comma-separated list of key-value pairs:

```tablegen
def MyType : TypeDef<MyDialect, "MyType"> {
  let parameters = (ins "int":$one, "int":$two, "int":$three);
  let mnemonic = "my_attr";
  let assemblyFormat = "`<` $three `:` struct($one, $two) `>`";
}
```

Use `struct(*)` to capture all parameters.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D111594