Fix DSLX ToBits derivation for arrays of tuples and empty tuples - #5300

Deriving ToBits for a DSLX struct could crash the frontend when a field contained an array of tuples or a nested empty tuple.

The tuple branch now uses the element type selected for the current expression. Array elements share a single type entry, so the second element of (bool,)[2] previously read past that entry. Concatenating zero elements now produces uN[0]:0, which handles () inside an aggregate and preserves the existing result for an empty struct.

The regression tests check that [(true,), (false,)] packs to u2:0b10 and that ((), bool) packs to one bit for either Boolean value.

Validation

• All 21 tests in //xls/dslx/type_system_v2:typecheck_module_v2_trait_test passed in an optimized CI build.
• The DSLX bytecode interpreter and XLS IR JIT both passed all Boolean input combinations for the two reported shapes.
• Ran clang-format on both changed C++ files, preserving the existing gMock and gTest include order. The XLS build cleaner and Bant passed for both affected targets.
• Ran the optimized //xls/... wildcard on b50d71752. All 2,205 selected test targets passed (1,074 executed, 1,131 cached). The sweep excluded //xls/contrib/..., //xls/dev_tools/..., //xls/visualization/..., and five tests whose dependency graphs reach OpenROAD, Yosys, or Clang.