Make clang-format remove duplicate headers when sorting #includes.

Make clang-format remove duplicate headers when sorting #includes.

Summary: When sorting #includes, #include directives that have the same text will be deduplicated when sorting #includes, and only the first #include in the duplicate #includes remains. If the `Cursor` is provided and put on a deleted #include, it will be put on the remaining #include in the duplicate #includes.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D23274

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@278206 91177308-0d34-0410-b5e6-96231b3b80d8