LLVM libc++ directories layout should reflect the same as in Google's NDK
Google's NDK includes just one version of LLVM libc++, while we include several. Our LLVM libc++ instances are located in
$NDK/sources/cxx-stl/llvm-libc++/$LLVM_VERSION, whereas in Google's NDK it reside in
$NDK/sources/cxx-stl/llvm-libc++ (note absent $LLVM_VERSION at the end). Unfortunately, Android Studio gradle plugin looks in hard-coded path
$NDK/sources/cxx-stl/llvm-libc++ when specifying
c++_shared as C++ Standard Library implementation. MS Visual Studio do the same. So we should keep the same directory layout (keeping there "default" LLVM libc++) to make AS and VS happy, even though it looks ugly.