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

[Joseph Huber via cfe-commits]

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@@ -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)
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jhuber6 wrote:

Not necessary for now, was just thinking how if we started adding new ones like `musl` or `glibc` then we'd need to disambiguate. Right now we probably just want `system` as the default and `llvm-libc` which mirrors this patch's binary selection well enough.

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