[clang-format] support header deletion in cleanupAroundReplacemnts.

[clang-format] support header deletion in cleanupAroundReplacemnts.

Summary:
- If a replacement has offset UINT_MAX, length 0, and a replacement text
  that is an #include directive, this will insert the #include into the
  correct block in the \p Code.
- If a replacement has offset UINT_MAX, length 1, and a replacement text
  that is the name of the header to be removed, the header will be removed
  from \p Code if it exists.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D24829

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282253 91177308-0d34-0410-b5e6-96231b3b80d8