[llvm] [lldb] [lld] [libc] [clang] [libcxx] [flang] [CMAKE] Enable FatLTO as a build option for LLVM (PR #80480)

[Paul Kirth via llvm-commits]

ilovepi wrote:

This speeds up a 2 stage build by about 3 minutes, tests time are still much closer than expected. Locally I saw a bout a 5-8% speedup to build clang unit tests, but maybe the unit tests are too small in the overall test regime to make a difference.

I'll try to collect more performance numbers locally w.r.t. the test times.

https://github.com/llvm/llvm-project/pull/80480