[libcxx] Support building hermetic static library

This is useful when static libc++ library is being linked into
shared libraries that may be used in combination with libraries.
We want to avoid we exporting libc++ symbols in those cases where
this option is useful. This is provided as a CMake option and can
be enabled by libc++ vendors as needed.

Differential Revision: https://reviews.llvm.org/D55404

llvm-svn: 350489