Unlock LTO

[kateinoigakukun]

This is an umbrella issue to track our work in high-level for link-time optimization (LTO)

Motivation

• Reduce startup time by reducing code size
  • Code size is a critical part of reducing initial (un-cached) startup time as it affects download size and compilation time inside Wasm engine.
• Gain more performance speedup by utilizing cross module information

Sub-tasks

• Embed bitcode info in stdlib and corelibs object files
• Adjust SwiftPM and swift-driver to properly propagate LTO options
• TBD