Fix formatting of complex #if expressions.

Before:
  #if !defined(AAAAAAAAAAAAAAAA) && (defined CCCCCCCC ||                         \
                                     defined DDDDDDDD) && defined(BBBBBBBB)

After:
  #if !defined(AAAAAAAAAAAAAAAA) && (defined CCCCCCCC || defined DDDDDDDD) &&    \
      defined(BBBBBBBB)

This fixes llvm.org/PR15828.