Add option to remove empty function exports

[GtechGovind]

Fixes #6506.

Summary

• add an opt-in remove-unused-module-elements-consider-empty-exports-unused pass argument
• remove exports of defined functions whose bodies are empty while preserving imported and non-empty exports
• let normal reachability analysis retain empty functions that are still used internally

Testing

• full CMake build
• Binaryen lit suite: 984 passed, 6 unsupported
• C++ unit suite: 420 passed
• legacy wasm-opt suite
• focused empty-export lit regression test

[kripken]

@orsinium , this is for a request you made over 2 years ago in #6506. Is it still relevant for you?

[orsinium]

It is! Firefly Zero is still a thing and thriving. Thank you :)