[clang] [CMake][Release] Build with -ffat-lto-objects (PR #140381)

[Nikita Popov via cfe-commits]

nikic wrote:

> How much slower is this? Last time I tested fatlto in our toolchain it took a VERY long time to link.

You're probably thinking about "full" LTO, this is fat thin LTO. Our naming is truly amazing. See https://llvm.org/docs/FatLTO.html.

It should add about 10% overhead.

https://github.com/llvm/llvm-project/pull/140381