clang-format: Support assignments as conditional operands.

Before:
  return a != b
             // comment
             ? a
             : a = a != b
                   // comment
         ? a =
               b : a;
After:
  return a != b
             // comment
             ? a
             : a = a != b
                       // comment
                       ? a = b
                       : a;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221987 91177308-0d34-0410-b5e6-96231b3b80d8