clang-format: Don't accidentally move tokens into preprocessor directive.

This fixes llvm.org/PR17265.

Before:
  Foo::Foo()
  #ifdef BAR
      : baz(0)
  #endif {
  }

After:
  Foo::Foo()
  #ifdef BAR
      : baz(0)
  #endif
  {
  }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190861 91177308-0d34-0410-b5e6-96231b3b80d8