[Tooling] Do not restore working dir in ClangTool

Summary:
Resolve all relative paths before running the tool instead.

This fixes the usage of ClangTool in AllTUsExecutor. The executor will
try running multiple ClangTool instances in parallel with compile
commands that usually have the same working directory.

Changing working directory is a global operation, so we end up
changing working directory in the middle of running other actions,
which leads to spurious compile errors.

Reviewers: ioeric, sammccall

Reviewed By: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51407

llvm-svn: 340937