Repair invalid DWARF scope ranges after transforms

[cpunion]

Summary

• preserve LLVM nonzero tombstones instead of remapping dead DIEs to address zero
• reject lost or reversed low/high PC pairs without unsigned wraparound
• normalize updated range lists and rebuild range-list parents from surviving child scopes
• make ambiguous overlapping sibling scopes unavailable instead of assigning them incorrect code ranges

Binaryen currently updates DWARF range endpoints independently. When optimization or Asyncify removes or reorders expressions, the resulting endpoints can wrap, overlap, or escape their parent scope. This is the same failure mode reported in #6406, extended to range lists and scope topology.

The repair is conservative: representable parent unions are preserved, while ambiguous scopes fail closed. Empty replacement range lists are appended rather than mutating lists that another DIE may share.

Validation

• python3 check.py wasm-opt
• all existing DWARF pass fixtures, including ignore_missing_func_dwarf, inlined_to_start_dwarf, and class_with_dwarf_noprint
• llvm-dwarfdump --verify reports no errors after roundtrip and Asyncify for those fixtures
• a 73-CU Emscripten final module that previously reported containment, internal-overlap, and sibling-overlap errors now verifies with no errors after its final Asyncify transform

Fixes #6406.