Move #include manipulation code to new lib/Tooling/Inclusions.

Summary:
clangToolingCore is linked into almost everything (incl. clang), but
not few tools need #include manipulation at this point. So pull this into a
separate library in Tooling.

Reviewers: ilya-biryukov

Subscribers: klimek, mgorny, cfe-commits, thakis

Differential Revision: https://reviews.llvm.org/D47068

llvm-svn: 332720