[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/instructions.rs

@@ -1296,7 +1296,7 @@ impl Into<InstSpec> for BoundInstruction {
 mod test {
     use super::*;
     use crate::cdsl::formats::InstructionFormatBuilder;
-    use crate::cdsl::operands::{OperandKindBuilder, OperandKindFields};
+    use crate::cdsl::operands::{OperandKind, OperandKindFields};
     use crate::cdsl::typevar::TypeSetBuilder;
     use crate::shared::types::Int::{I32, I64};
 
@@ -1304,7 +1304,7 @@ mod test {
         // Pretend the index string is &'static.
         let name = Box::leak(index.to_string().into_boxed_str());
         // Format's name / rust_type don't matter here.
-        let kind = OperandKindBuilder::new(name, name, field).build();
+        let kind = OperandKind::new(name, name, field);
         let operand = Operand::new(name, kind);
         operand
     }