[CMake][Win32.DLL] Let llvm_add_library(SHARED) link dependent libraries as PRIVATE.

For example, c-index-test.exe requires just libclang.dll (its import library).
When libraries in libclang were not PRIVATE but PUBLIC, c-index-test required libraries transitive by libclang.

Note, on mingw with BUILD_SHARED_LIBS, library dependencies would become more strict.
In principle, required libraries should be "required in its source file".

This will help to detect missing dependencies.