[clangd] Record the file being processed in a TUScheduler thread in context.

Summary:
This allows implementations like different symbol indexes to know what
the current active file is. For example, some customized index implementation
might decide to only return results for some files.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: javed.absar, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50446

llvm-svn: 339320