[Dexter] Add --source-dir-root flag

Summary:
This allows to run dexter tests with separately compiled
binaries that are specified via --binary if the source file
location changed between compilation and dexter test run.

Reviewers: TWeaver, jmorse, probinson, #debug-info

Reviewed By: jmorse

Subscribers: #debug-info, cmtice, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D81319