[CMake] LLVM_PLUGIN_EXT: Use CMAKE_SHARED_LIBRARY_SUFFIX rather than CMAKE_SHARED_MODULE_SUFFIX in llvm tree.

FIXME: llvm/test may be aware of LLVM_PLUGIN_EXT, like as clang/test does.
FIXME: CMAKE_*_SUFFIX may be set in HandleLLVMOptions if those variables could be writable, rather than to set one as target properties.
[CMake] llvm_add_library(MODULE) may use CMAKE_MODULE_LINKER_FLAGS instead of target property LINK_FLAGS.

I mis-dropped Darwin's link flags (in clang side) since r201073.
[CMake] Move the target property PREFIX from add_llvm_loadable_module() to llvm_add_library().