[Tooling] Restore working dir in ClangTool.

Summary:
And add an option to disable this behavior. The option is only used in
AllTUsExecutor to avoid races when running concurrently on multiple
threads.

This fixes PR38869 introduced by r340937.

Reviewers: ioeric, steveire

Reviewed By: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51864

llvm-svn: 341910