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

[via llvm-commits]

lntue 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.

Done.

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