PR libstdc++/88607 add tests using -finput-charset=ascii

PR libstdc++/88607 add tests using -finput-charset=ascii

This verifies that the <bits/extc++.h> header can be compiled with ASCII
as the input character set.

	PR libstdc++/88607
	* testsuite/17_intro/headers/c++1998/charset.cc: New test.
	* testsuite/17_intro/headers/c++2011/charset.cc: New test.
	* testsuite/17_intro/headers/c++2014/charset.cc: New test.
	* testsuite/17_intro/headers/c++2017/charset.cc: New test.
	* testsuite/17_intro/headers/c++2020/charset.cc: New test.

From-SVN: r267607