[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201316 91177308-0d34-0410-b5e6-96231b3b80d8
[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201317 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] Move the target property PREFIX from add_llvm_loadable_module() to llvm_add_library().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201318 91177308-0d34-0410-b5e6-96231b3b80d8