[CMake] add_clang_library(): Use llvm_add_library.

CMAKE_MODULE_LINKER_FLAGS can be removed since llvm_add_library(MODULE) adds same flags to MODULE.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201073 91177308-0d34-0410-b5e6-96231b3b80d8