CMake: Set HOST_LINK_VERSION on Darwin (PR21268)

The Autoconf build already does this, but it was never ported to
CMake. The host linker version affects the flags that Clang pass
to the linker, notably whether it passes -demangle or not.

http://reviews.llvm.org/D6239

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221844 91177308-0d34-0410-b5e6-96231b3b80d8