[libclang] Link clang-tidy plugin into libclang if present.

This is a sad workaround for the lack of plugin support in libclang. Depends
on D17807, a tools-extra change that also contains the test case.

This is designed to be easy to remove again if libclang ever grows proper
plugin support.

Differential Revision: http://reviews.llvm.org/D17808

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262596 91177308-0d34-0410-b5e6-96231b3b80d8