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 config parameter to LTO backend, specifying which LTO mode is desired when using unified LTO.
- Add unified LTO flag to the summary index for efficiency. Unified LTO modules can be detected without parsing the module.
- Make sure that the ModuleID is generated by incorporating more types of symbols.