clang-format: Understand sequenced casts.

This fixed llvm.org/PR20712.

Before:
  int i = (int)(int) -2;

After:
  int i = (int)(int)-2;

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216378 91177308-0d34-0410-b5e6-96231b3b80d8