[llvm] [llvm][cmake] Do not emit error on `libc`'s use of project + runtime build (PR #110038)

[Michael Jones via llvm-commits]

https://github.com/michaelrj-google approved this pull request.

Fine for now, but I hope that soon we'll be able to fully remove old headergen and get rid of these exceptions.

https://github.com/llvm/llvm-project/pull/110038