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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214966 91177308-0d34-0410-b5e6-96231b3b80d8