clang-format: Improve formatting of operators forced to new lines.

Before:
  unsigned ContentSize =
      sizeof(int16_t) // DWARF ARange version number
      +
      sizeof(int32_t) // Offset of CU in the .debug_info section
      +
      sizeof(int8_t) // Pointer Size (in bytes)
      +
      sizeof(int8_t); // Segment Size (in bytes)

After:
  unsigned ContentSize =
      sizeof(int16_t)   // DWARF ARange version number
      + sizeof(int32_t) // Offset of CU in the .debug_info section
      + sizeof(int8_t)  // Pointer Size (in bytes)
      + sizeof(int8_t); // Segment Size (in bytes)

This fixes llvm.org/PR17687.

llvm-svn: 194276