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

[Nikita Popov via cfe-commits]

nikic wrote:

Looking at our current cmake config, we're using:
```
-DLIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY=ON \
-DLIBCXX_ENABLE_ABI_LINKER_SCRIPT=ON \
```
Do I understand correctly that the new replacement for that would be?
```
-DLIBCXX_ABILIB_FOR_SHARED="shared-libcxxabi" \
-DLIBCXX_ABILIB_FOR_STATIC="merged-libcxxabi" \
-DLIBCXX_ENABLE_ABI_LINKER_SCRIPT=ON \
```
And those LIBCXX_ABILIB options are also the defaults, if I'm reading the docs right.

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