libstdc++: Remove redundant 17_intro/headers tests

We have several nearly identical tests under 17_intro/headers which only
differ in a -std option set using dg-options. Since the testsuite now
supports running tests with multiple -std options (and I test that
regularly) we don't need these duplicated tests. We can remove most of
them and let the testsuite decide which -std option to use.

In the all_attributes.cc case the content of the tests is slightly
different, but they can be combined into one test that defines macros
conditionally based on __cplusplus checks.

The stdc++.cc tests could also be combined this way, but for now I've
just kept one version for c++98 and one for all later standards.

For stdc++_multiple_inclusion.cc we can remove the body of the files and
just include stdc++.cc twice. This means we don't need to add includes
to both stdc++.cc and stdc++_multiple_inclusion.cc, we only need to
update one place.

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/headers/c++1998/all_attributes.cc: Add
	attribute names from later standards and remove dg-options.
	* testsuite/17_intro/headers/c++1998/stdc++.cc: Add c++98_only
	target selector.
	* testsuite/17_intro/headers/c++1998/stdc++_multiple_inclusion.cc:
	Remove content and include stdc++.cc twice instead.
	* testsuite/17_intro/headers/c++2011/stdc++.cc: Replace
	dg-options with c++11 target selector.
	* testsuite/17_intro/headers/c++2011/stdc++_multiple_inclusion.cc:
	Remove content and include stdc++.cc twice instead.
	* testsuite/17_intro/headers/c++2011/all_attributes.cc: Removed.
	* testsuite/17_intro/headers/c++2011/all_no_exceptions.cc: Removed.
	* testsuite/17_intro/headers/c++2011/all_no_rtti.cc: Removed.
	* testsuite/17_intro/headers/c++2011/all_pedantic_errors.cc: Removed.
	* testsuite/17_intro/headers/c++2011/charset.cc: Removed.
	* testsuite/17_intro/headers/c++2011/operator_names.cc: Removed.
	* testsuite/17_intro/headers/c++2014/all_attributes.cc: Removed.
	* testsuite/17_intro/headers/c++2014/all_no_exceptions.cc: Removed.
	* testsuite/17_intro/headers/c++2014/all_no_rtti.cc: Removed.
	* testsuite/17_intro/headers/c++2014/all_pedantic_errors.cc: Removed.
	* testsuite/17_intro/headers/c++2014/charset.cc: Removed.
	* testsuite/17_intro/headers/c++2014/operator_names.cc: Removed.
	* testsuite/17_intro/headers/c++2014/stdc++.cc: Removed.
	* testsuite/17_intro/headers/c++2014/stdc++_multiple_inclusion.cc: Removed.
	* testsuite/17_intro/headers/c++2017/all_attributes.cc: Removed.
	* testsuite/17_intro/headers/c++2017/all_no_exceptions.cc: Removed.
	* testsuite/17_intro/headers/c++2017/all_no_rtti.cc: Removed.
	* testsuite/17_intro/headers/c++2017/all_pedantic_errors.cc: Removed.
	* testsuite/17_intro/headers/c++2017/charset.cc: Removed.
	* testsuite/17_intro/headers/c++2017/operator_names.cc: Removed.
	* testsuite/17_intro/headers/c++2017/stdc++.cc: Removed.
	* testsuite/17_intro/headers/c++2017/stdc++_multiple_inclusion.cc: Removed.
	* testsuite/17_intro/headers/c++2020/all_attributes.cc: Removed.
	* testsuite/17_intro/headers/c++2020/all_no_exceptions.cc: Removed.
	* testsuite/17_intro/headers/c++2020/all_no_rtti.cc: Removed.
	* testsuite/17_intro/headers/c++2020/all_pedantic_errors.cc: Removed.
	* testsuite/17_intro/headers/c++2020/charset.cc: Removed.
	* testsuite/17_intro/headers/c++2020/operator_names.cc: Removed.
	* testsuite/17_intro/headers/c++2020/stdc++.cc: Removed.
	* testsuite/17_intro/headers/c++2020/stdc++_multiple_inclusion.cc: Removed.