diagnostic: avoid repeating include path

diagnostic: avoid repeating include path

When a sequence of diagnostic messages bounces back and forth repeatedly
between two includes, as with

 #include <map>
 std::map<const char*, const char*> m ("123", "456");

The output is quite a bit longer than necessary because we dump the include
path each time it changes.  I'd think we could print the include path once
for each header file, and then expect that the user can look earlier in the
output if they're wondering.

gcc/ChangeLog:

	* diagnostic.h (struct diagnostic_context): Add includes_seen.
	* diagnostic.c (diagnostic_initialize): Initialize it.
	(diagnostic_finish): Clean it up.
	(includes_seen): New function.
	(diagnostic_report_current_module): Use it.

gcc/testsuite/ChangeLog:

	* c-c++-common/cpp/line-2.c: Only expect includes once.
	* c-c++-common/cpp/line-3.c: Likewise.