[clang-format] Refactor #include insertion/deletion functionality into a class.

Summary:
The class will be moved into libToolingCore as followup.

The new behaviors in this patch:
- New #include is inserted in the right position in a #include block to
preserver sorted #includes. This is best effort - only works when the
block is already sorted.
- When inserting multiple #includes to the end of a file which doesn't
end with a "\n" character, a "\n" will be prepended to each #include.
This is a special and rare case that was previously handled. This is now
relaxed to avoid complexity as it's rare in practice.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, cfe-commits, djasper

Differential Revision: https://reviews.llvm.org/D46180

llvm-svn: 331544