[libc-commits] [libc] [libc] raise CMAKE_CXX_STANDARD to C++20 (PR #87020)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

Further discussions today; it sounds like libc++ was able to move away from the `DLLVM_ENABLE_PROJECTS` and now only supports `DLLVM_ENABLE_RUNTIMES`:
- documentation change: https://reviews.llvm.org/D111356
- explicit warning: https://reviews.llvm.org/D112724
- removal: https://reviews.llvm.org/D132480

This might be tricky for us because of host dependencies such as:
- libc-hdrgen
- gpu server/utilities

Though it sounds like a have a "bootstrap cross build" https://libc.llvm.org/full_cross_build.html#bootstrap-cross-build where this might not be a problem?

cc @petrhosek 

https://github.com/llvm/llvm-project/pull/87020