[clang] [llvm] [fatlto] Use the ThinLTO default pipeline for FatLTO (PR #134434)

[Paul Kirth via llvm-commits]

ilovepi wrote:

hmm, I tried adding some of the individual passes, and ran into some other errors. Let me give that another try w/ that PR as an example. 

https://github.com/llvm/llvm-project/pull/134434