[Refactor] Moving SourceExtraction header from lib to include

Summary:
- Moved the SourceExtraction header from lib to include so that it can be used in clangd.

Reviewers: arphaman

Subscribers: ilya-biryukov, dexonsmith, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65878

llvm-svn: 368743