Get less confused by trailing comma in Google style.

The formatter can now format:
void aaaaaaaaaaaaaaaaaa(int level,
                        double *min_x,
                        double *max_x,
                        double *min_y,
                        double *max_y,
                        double *min_z,
                        double *max_z, ) {
}

Although this is invalid code, it frequently happens during development and
clang-format should be nicer :-).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175151 91177308-0d34-0410-b5e6-96231b3b80d8