[CMake] add_clang_library(): Honor STATIC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200759 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] add_clang_library(): Fix LLVM_PLUGIN_EXT in MODULE. It was broken since my r199902.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200760 91177308-0d34-0410-b5e6-96231b3b80d8