[lldb] Check that files exists before handing them off to FileCollector

The FileCollector asserts that paths passed to addDirectory are indeed
directories. For that to work, the file needs to actually exist. In the
downstream Swift fork we have tests that remove files during testing,
which resulted in this assertion getting triggered.

GitOrigin-RevId: 31fa76fd24adea755e4368674a59385d2c5b0cc7
1 file changed