[meta] Remove the OperandKindBuilder;

And replace it by constructors in OperandKind. There's a single optional
parameter function `set_doc` that remains, and didn't justify the whole
OperandKindBuilder concept to exist.

cranelift/codegen/meta/src/cdsl/formats.rs

@@ -49,6 +49,7 @@ pub(crate) struct InstructionFormat {
 pub(crate) struct FormatStructure {
     pub num_value_operands: usize,
     pub has_value_list: bool,
+    /// Tuples of (Rust field name / Rust type) for each immediate field.
     pub imm_field_names: Vec<(&'static str, &'static str)>,
 }