clang-format: Add special comments to disable formatting.

With this patch:
  int ThisWillBeFormatted;
  // clang-format off
  int   ThisWontBeFormatted;
  // clang-format on
  int Formatted;

This is for regions where a significantly nicer code layout can be found
knowing the content of the code.

This fixes llvm.org/PR20463.

llvm-svn: 214966