Here's a high level summary of the changes in this patch. For more information
on rational, see the RFC (https://discourse.llvm.org/t/rfc-a-unified-lto-bitcode-frontend/61774).
- Add flags to the clang driver to enable the unified LTO pipeline.
- When this mode is enabled, add the "UnifiedLTO" module flag to any bitcode modules generated.
- Use the Full (Regular) LTO passes to optimize LTO bitcode modules in the frontend when using unified LTO.
- Emit type metadata when this pipeline is enabled.
- Add LLD driver changes to this patch.