clang-format: Introduce DisableFormat that prevents formatting.

And "none" pseudo-style indicating that formatting should be not
applied.

(1) Using .clang-format with "DisableFormat: true" effectively prevents
formatting for all files within the folder containing such .clang-format
file.

(2) Using -fallback-style=none together with -style=file prevents
formatting when .clang-format is not found, which can be used in on-save
callback.

Patch by Adam Strzelecki. Thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209446 91177308-0d34-0410-b5e6-96231b3b80d8