[CMake] Fix the setting of LIBCXX_HEADER_DIR
[CMake] Fix the setting of LIBCXX_HEADER_DIR

Reviewers: phosek

Subscribers: mgorny, christof, ldionne, cfe-commits

Differential Revision: https://reviews.llvm.org/D49629

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@337727 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx