DWARF member-location evaluator implements only a subset of DW_OP

Status (2026-10-17): deliberate limitation, recorded with the fix that made it visible.

abicheck/dwarf_utils.py evaluates DW_AT_data_member_location expressions with a small stack machine. It implements DW_OP_plus, DW_OP_plus_uconst, DW_OP_constu, DW_OP_consts and DW_OP_lit0..31. Before this change, any other opcode was either skipped or pushed as a constant, so the member offset silently came out wrong. Now a member location that uses an unsupported opcode, or an underflowing, truncated or otherwise malformed expression, yields "unknown" (None). The member's field offset is then unknown, and the enclosing record's layout is not recorded when that offset is needed.

What is not done: the unsupported operations (DW_OP_minus, DW_OP_mul, DW_OP_deref, DW_OP_shl, and the rest) are not implemented. Mainstream compilers do not put them in data_member_location, so the practical effect is limited to unusual toolchains. Layouts that depend on them lose field-offset evidence, which lowers assurance and never produces a clean result or a fabricated break.

Implementing the remaining operations would be a feature, not a fix. It needs a real fixture that emits them and an oracle that is not this evaluator.