[CMake] Install libcxx-headers as part of the Apple-stage2 distribution

This installs the clang headers as part of the install-distribution target.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268320 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/caches/Apple-stage2.cmake

@@ -55,6 +55,7 @@ set(LLVM_DISTRIBUTION_COMPONENTS
   LTO
   clang-format
   clang-headers
+  libcxx-headers
   ${LLVM_TOOLCHAIN_TOOLS}
   CACHE STRING "")