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.