selftest: split out named_temp_file from temp_source_file

selftest: split out named_temp_file from temp_source_file

Split out a new base class for temp_source_file, named_temp_file,
moving the deletion to the base class dtor, so that we can write
out temporary files in other ways in selftests.

gcc/ChangeLog:
	* selftest.c (selftest::named_temp_file::named_temp_file): New
	ctor.
	(selftest::temp_source_file::~temp_source_file): Move to...
	(selftest::named_temp_file::~named_temp_file): ...here.
	(selftest::test_named_temp_file): New function.
	(selftest::selftest_c_tests): Call test_named_temp_file.
	* selftest.h (class named_temp_file): New class.
	(class temp_source_file): Convert to a subclass of named_temp_file.

From-SVN: r239875