[clang] [libc] [libcxx] [libc][libcxx] Support for building libc++ against LLVM libc (PR #99287)

[Louis Dionne via cfe-commits]

================
@@ -293,6 +293,7 @@ option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
+option(LIBCXX_USE_LLVM_LIBC "Build libc++ against LLVM libc." OFF)
----------------
ldionne wrote:

I would like to avoid introducing a CMake setting like this. Instead, we should do something similar to the way we handle ABI choices and provide a pseudo-target that represents the libc we're building against. We can then have a multi-valued option like `LIBCXX_LIBC="llvm-libc|system-libc|musl|etc..."`.

https://github.com/llvm/llvm-project/pull/99287