Make ArgumentsAdjuster an std::function (clang-tools-extra part of D6505).

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6506

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@223249 91177308-0d34-0410-b5e6-96231b3b80d8