[llvm] [DTLTO] [LLVM] Initial DTLTO cache implementation (PR #156433)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Sorry for the slow review. Comments below.

Please also add llvm-lto2 based tests to llvm/test/ThinLTO/X86/dtlto, so that this gets exercised by LLVM tests.

https://github.com/llvm/llvm-project/pull/156433