Currently it's possible to select whether to statically link unwinder or the C++ ABI library, but this option applies to both the shared and static library. However, in some scenarios it may be desirable to only statically link unwinder and C++ ABI library into static C++ library since for shared C++ library we can rely on dynamic linking and linker scripts. This change enables selectively enabling or disabling statically linking only to shared or static library. Differential Revision: https://reviews.llvm.org/D49502 git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337668 91177308-0d34-0410-b5e6-96231b3b80d8