Use z instead of ZLIB::ZLIB

[lxfind]

Summary

With D79219, when building LLVM, the following is seen in file build_dir/lib/cmake/llvm/LLVMExports.cmake:

set_target_properties(LLVMSupport PROPERTIES
  INTERFACE_LINK_LIBRARIES "curses;m;ZLIB::ZLIB;LLVMDemangle"

We should be consistent in how to set import libs here. Everywhere else we use the short names, such as rt, m and etc.
We should also use z here.

[arsenm]

This seems like the backwards direction. Namespaced imported targets are the more modern cmake option

[lxfind]

@arsenm, we use the direct names everywhere else though (if you scan through the rest of this file). Anyway, I don't mind using it as long as it works properly. I have seen (and have seen others asking in the other commit as well), that ZLIB::ZLIB shows up in build_dir/lib/cmake/llvm/LLVMExports.cmake. That needs to be resolved.

[phosek]

I agree with @arsenm, this is backwards of what we want to do. The goal of D79219 is to move to modern CMake and simplify life for consumers of LLVM like Swift.

llvm/lib/Support/CMakeLists.txt

 if(LLVM_ENABLE_ZLIB)
-  set(imported_libs ZLIB::ZLIB)
+  set(imported_libs z)
 endif()
 
 if( MSVC OR MINGW )
   # libuuid required for FOLDERID_Profile usage in lib/Support/Windows/Path.inc.
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.
   set(system_libs ${system_libs} psapi shell32 ole32 uuid advapi32)
 elseif( CMAKE_HOST_UNIX )
   if( HAVE_LIBRT )