Add -Werror flag to clang copts

Use -Wno-unused-command-line-argument to suppress the unused {MD, MT,
MF} flags. It is also applied in
https://github.com/google/iree/blob/75d4bb8/build_tools/cmake/iree_copts.cmake#L105

Change-Id: I31dc29ad273dae2123744299e98e802c5e453551
1 file changed