libcpp: Fix __has_include_next ICE in the last directory of the path [PR80755]

In libcpp/files.cc, the function _cpp_has_header(), which implements
__has_include and __has_include_next, does not check for a NULL return value
from search_path_head(), leading to an ICE tripping an assert when
_cpp_find_file() tries to use it. Fix it by checking for that case and
silently returning false instead.

As suggested by the PR author, it is easiest to make a testcase by using
the -idirafter option. To enable that, also modify the dg-additional-options
testsuite procedure to make the global $srcdir available, since -idirafter
requires the full path.

libcpp/ChangeLog:

	PR preprocessor/80755
	* files.cc (search_path_head): Add SUPPRESS_DIAGNOSTIC argument
	defaulting to false.
	(_cpp_has_header): Silently return false if the search path has been
	exhausted, rather than issuing a diagnostic and then hitting an
	assert.

gcc/testsuite/ChangeLog:

	* lib/gcc-defs.exp (dg-additional-options): Make $srcdir usable in a
	dg-additional-options directive.
	* c-c++-common/cpp/has-include-next-2-dir/has-include-next-2.h: New test.
	* c-c++-common/cpp/has-include-next-2.c: New test.

gcc/testsuite/c-c++-common/cpp/has-include-next-2-dir/has-include-next-2.h

@@ -0,0 +1,3 @@
+#if __has_include_next(<whatever>)
+/* This formerly led to an ICE when the current directory was the last one in the path.  */
+#endif

gcc/testsuite/c-c++-common/cpp/has-include-next-2.c

@@ -0,0 +1,4 @@
+/* PR preprocessor/80755 */
+/* { dg-do preprocess } */
+/* { dg-additional-options "-idirafter $srcdir/c-c++-common/cpp/has-include-next-2-dir" } */
+#include <has-include-next-2.h>

gcc/testsuite/lib/gcc-defs.exp

@@ -280,6 +280,7 @@ if { [info exists env(GCC_RUNTEST_PARALLELIZE_DIR)] \
 
 proc dg-additional-options { args } {
     upvar dg-extra-tool-flags extra-tool-flags
+    global srcdir
 
     if { [llength $args] > 3 } {
 	error "[lindex $args 0]: too many arguments"

libcpp/files.cc

@@ -181,7 +181,8 @@ static bool read_file_guts (cpp_reader *pfile, _cpp_file *file,
 static bool read_file (cpp_reader *pfile, _cpp_file *file,
 		       location_t loc);
 static struct cpp_dir *search_path_head (cpp_reader *, const char *fname,
-				 int angle_brackets, enum include_type);
+					 int angle_brackets, enum include_type,
+					 bool suppress_diagnostic = false);
 static const char *dir_name_of_file (_cpp_file *file);
 static void open_file_failed (cpp_reader *pfile, _cpp_file *file, int,
 			      location_t);
@@ -1041,7 +1042,7 @@ _cpp_mark_file_once_only (cpp_reader *pfile, _cpp_file *file)
    nothing left in the path, returns NULL.  */
 static struct cpp_dir *
 search_path_head (cpp_reader *pfile, const char *fname, int angle_brackets,
-		  enum include_type type)
+		  enum include_type type, bool suppress_diagnostic)
 {
   cpp_dir *dir;
   _cpp_file *file;
@@ -1070,7 +1071,7 @@ search_path_head (cpp_reader *pfile, const char *fname, int angle_brackets,
     return make_cpp_dir (pfile, dir_name_of_file (file),
 			 pfile->buffer ? pfile->buffer->sysp : 0);
 
-  if (dir == NULL)
+  if (dir == NULL && !suppress_diagnostic)
     cpp_error (pfile, CPP_DL_ERROR,
 	       "no include path in which to search for %s", fname);
 
@@ -2164,7 +2165,10 @@ bool
 _cpp_has_header (cpp_reader *pfile, const char *fname, int angle_brackets,
 		 enum include_type type)
 {
-  cpp_dir *start_dir = search_path_head (pfile, fname, angle_brackets, type);
+  cpp_dir *start_dir = search_path_head (pfile, fname, angle_brackets, type,
+					 /* suppress_diagnostic = */ true);
+  if (!start_dir)
+    return false;
   _cpp_file *file = _cpp_find_file (pfile, fname, start_dir, angle_brackets,
 				    _cpp_FFK_HAS_INCLUDE, 0);
   return file->err_no != ENOENT;