clang-format: Improve handling of comments in binary expressions.

Before:
  b = a &&
      // Comment
      b.c &&
      d;

After:
  b = a &&
      // Comment
      b.c && d;

This fixes llvm.org/PR21535.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221727 91177308-0d34-0410-b5e6-96231b3b80d8