[CMake] Create a separate install target for libcxx headers

This change doesn't impact the behavior of the install-libcxx target which installs whichever libcxx components you build, it just adds a separate target to just install the headers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@268124 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed