[llvm] [libc][cmake] Make libc not included in LLVM_ENABLE_PROJECTS=all. (PR #142626)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

A few lines later, we have "Using LLVM_ENABLE_PROJECTS=libc is deprecated now, and will become a fatal error in the LLVM 21 release."  If we're going to make it a fatal error for 21, we should probably do that soon... if not, we should update the text with an updated timeline.

https://github.com/llvm/llvm-project/pull/142626