[Tooling] Pull #include manipulation code from clangFormat into libToolingCore.

Summary: Also pull #include related style out of FormatStyle as tooling::IncludeStyle.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, mgorny, cfe-commits, djasper

Differential Revision: https://reviews.llvm.org/D46496

llvm-svn: 332287