Fix libcxxabi CMake detection of libcxx path to handle LLVM_EXTERNAL_LIBCXX_SOURCE_DIR flag
Fix libcxxabi CMake detection of libcxx path to handle LLVM_EXTERNAL_LIBCXX_SOURCE_DIR flag

Differential Revision: https://reviews.llvm.org/D26364

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@286161 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi