build-libcxx: Stop disabling libc++experimental

Since LLVM 15, libc++experimental should work fine with libc++
configured for shared linking, and libc++experimental is enabled
unconditionally, and LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY is
ignored now.

build-libcxx.sh

@@ -99,7 +99,6 @@ for arch in $ARCHS; do
         -DLIBCXX_USE_COMPILER_RT=ON \
         -DLIBCXX_ENABLE_SHARED=$BUILD_SHARED \
         -DLIBCXX_ENABLE_STATIC=$BUILD_STATIC \
-        -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=OFF \
         -DLIBCXX_ENABLE_STATIC_ABI_LIBRARY=TRUE \
         -DLIBCXX_CXX_ABI=libcxxabi \
         -DLIBCXX_LIBDIR_SUFFIX="" \