[CMake] Fix the LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY option

This option should be available if LIBCXX_ENABLE_SHARED is enabled,
not LIBCXX_ENABLE_STATIC.

This fixes a typo from SVN r337814.

Differential Revision: https://reviews.llvm.org/D50691

llvm-svn: 339697