[debuginfo-tests] Warn, not error, if we can't delete working directory

On Windows, an error running the debugger typically leaves a process
hanging around in the working directory. When Dexter exits, it can't then
delete the working directory and produces an exception, masking the problem
in the debugger. (This can be worked around by specifying --save-temps).
Rather than hard-erroring, print a warning when we can't delete the working
directory instead.

It'd be much better to improve our error handling, and make the
WorkingDirectory class aware that something's wrong when it enters exit.
However, this is something that's going to mask genuine errors and make
everyones lives harder right now, so I think this non-ideal fix is
important to get in first.

Differential Revision: https://reviews.llvm.org/D74548