Revert "[libc++] Only add dylib-related features when using the system's libc++"
Revert "[libc++] Only add dylib-related features when using the system's libc++"

This reverts r353319, which broke our internal CI.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@353321 91177308-0d34-0410-b5e6-96231b3b80d8

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