Improve fallback names for imported and exported items - #9079

Fixes #2504

Summary

• use import field names as fallbacks for imported items without explicit name-section entries
• pre-scan exports and use the first export name as the fallback for unnamed defined items
• preserve explicit name-section priority and deduplicate fallback names through the existing naming logic

Testing

• cmake --build build --target wasm-as wasm-dis wasm-opt -j4
• PATH=/tmp/binaryen-sep6-venv/bin:$PATH /tmp/binaryen-sep6-venv/bin/python build/bin/binaryen-lit test/lit/binary -j4 (21 passed)