analyzer: fix tests for UNKNOWN_LOCATION

analyzer: fix tests for UNKNOWN_LOCATION

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

    |   15 |   while (fgets(buf, 10, fp) != NULL)
    |      |         ~
    |      |         |
    |      |         (2) following 'false' branch...
    |
  'f1': event 3
    |
    |cc1:
    |
  'f1': event 4
    |
    |<source>:19:1:
    |   19 | }
    |      | ^
    |      | |
    |      | (4) 'fp' leaks here; was opened at (1)
    |

The root cause is that various places in the analyzer compare locations
against UNKNOWN_LOCATION, which 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 whenever testing
against UNKNOWN_LOCATION to look through ad-hoc wrappers.

For the case above, it thus picks a better location in
supernode::get_start_location for event (3) above, improving it to:

    |   15 |   while (fgets(buf, 10, fp) != NULL)
    |      |         ~
    |      |         |
    |      |         (2) following 'false' branch...
    |......
    |   19 | }
    |      | ~
    |      | |
    |      | (3) ...to here
    |      | (4) 'fp' leaks here; was opened at (1)
    |

gcc/analyzer/ChangeLog:
	PR analyzer/58237
	* engine.cc (leak_stmt_finder::find_stmt): Use get_pure_location
	when comparing against UNKNOWN_LOCATION.
	(stmt_requires_new_enode_p): Likewise.
	(exploded_graph::dump_exploded_nodes): Likewise.
	* supergraph.cc (supernode::get_start_location): Likewise.
	(supernode::get_end_location): Likewise.

gcc/testsuite/ChangeLog:
	PR analyzer/58237
	* gcc.dg/analyzer/file-paths-1.c: New test.