[clang] [compiler-rt] [libcxx] [llvm] [libc++] Replace LIBCXX_ENABLE_STATIC_ABI_LIBRARY & friends by a new LIBCXX_CXX_ABI choice (PR #112978)

[Louis Dionne via cfe-commits]

ldionne wrote:

@petrhosek I'm curious to know what you think of this design.

https://github.com/llvm/llvm-project/pull/112978