[dsymutil] Warn on timestmap mismatch between object file and debug map

This re-lands e5553b9a with two small fixes to the tests:

 - Don't touch the source directory in debug-map-parsing.test but
   instead copy everything over in a temporary directory in
   timestamp-mismatch.test.
 - Don't redirect stderr to stdout to avoid the output getting
   intertwined in extern-alias.test.