[llvm] A Unified LTO Bitcode Frontend

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.

Differential Revision: https://reviews.llvm.org/D123803