[clang-tidy] Share the forced linking code between clang-tidy tool and plugin

Extract code that forces linking to the separate header and include it in both plugin and standalone tool.
Try 2: missing header guard and "clang/Config/config.h" are added to the new header.

Differential Revision: https://reviews.llvm.org/D55595

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@349131 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed