tree-diagnostic-path.cc: properly handle ad-hoc wrappers of UNKNOWN_LOCATION

tree-diagnostic-path.cc: properly handle ad-hoc wrappers of UNKNOWN_LOCATION

In the reproducer for PR analyzer/58237 I noticed that some events that
were missing locations were also missing text; for example event 3 here:

    |   15 |   while (fgets(buf, 10, fp) != NULL)
    |      |         ~
    |      |         |
    |      |         (2) following 'false' branch...
    |
  'f1': event 3
    |
    |cc1:
    |

The root cause is that the path_summary-printing code doesn't consider
ad-hoc locations when looking for reserved locations, and so fails to
detect an unknown location for the case where an unknown location has
been wrapped into an ad-hoc location to record a block.

This patch fixes the issue by using get_pure_location, thus looking
through ad-hoc wrappers, improving the result to:

    |   15 |   while (fgets(buf, 10, fp) != NULL)
    |      |         ~
    |      |         |
    |      |         (2) following 'false' branch...
    |
  'f1': event 3
    |
    |cc1:
    | (3): ...to here
    |

gcc/ChangeLog:
	* tree-diagnostic-path.cc (path_summary::event_range::print):
	When testing for UNKNOWN_LOCATION, look through ad-hoc wrappers
	using get_pure_location.