dslx: parse semantic sum declarations and constructors

[dank-openai]

Summary

• Add DSLX frontend support for semantic-sum enum declarations with unit, tuple, and struct payload variants.
• Add semantic-sum constructor expressions and match payload patterns so the AST preserves the source payload shape.
• Teach formatting, cloning, LSP symbols, type-info plumbing, conversion scaffolding, and diagnostics about the new AST nodes while leaving full typechecking and lowering semantics for follow-up work.
• Preserve existing numeric-enum parsing behavior, including explicitly typed empty enums.

Example

enum Option<T: type> {
  None,
  Some(T),
}

This PR makes declarations and constructors like Option::Some(x) parse and format as semantic-sum syntax. Later changes can build typechecking and conversion semantics on top of these AST nodes.

Validation

Validation ran on Ubuntu 24.04 at commit 1f8ad4299fa61d5d35d2bbd4fe4780f79314f2d8:

• bazel test //xls/dslx/fmt:ast_fmt_test //xls/dslx/frontend:parser_test //xls/dslx/frontend:ast_cloner_test //xls/dslx/frontend:scanner_test //xls/dslx:dslx_fmt_test //xls/dslx:constexpr_evaluator_test //xls/dslx/bytecode:bytecode_emitter_test //xls/dslx/exhaustiveness:exhaustiveness_match_test //xls/dslx/lsp:document_symbols_test //xls/dslx/translators:dslx_to_verilog_test //xls/fuzzer:value_generator_test //xls/dslx/ir_convert:ir_converter_test //xls/dslx/ir_convert:ir_converter_legacy_test //xls/dslx/tests/errors:error_modules_test //xls/dslx/run_routines:run_routines_test //xls/dslx/type_system:type_info_to_proto_test //xls/dslx/type_system_v2:typecheck_module_v2_enum_test --test_output=errors --keep_going --test_timeout=900 --cache_test_results=no

Misc.

This is PR 1/7 for phase 1 implementation of sum types.