[clang-tidy] Add an option to export suggested fixes into a file.

Allows gathering fixes and applying them with clang-apply-fixes.

Differential Revision: http://reviews.llvm.org/D5176

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217139 91177308-0d34-0410-b5e6-96231b3b80d8