c++: Add testcase for C++23 P2316R2 - consistent character literal encoding [PR102615]

c++: Add testcase for C++23 P2316R2 - consistent character literal encoding [PR102615]

I believe we need no changes to the compiler for P2316R2, seems we treat
character literals the same between preprocessor and C++ expressions,
here is a testcase that should verify it.

Note, seems the internal charset for GCC can be either UTF-8 or UTF-EBCDIC,
but I bet it is very hard (at least for me) to actually test the latter.
I'd guess one needs all system headers to be in EBCDIC and the gcc sources too.
But looking around the source, I'm a little bit worried about the UTF-EBCDIC
case.
One is:
 #if  '\n' == 0x0A && ' ' == 0x20 && '0' == 0x30 \
    && 'A' == 0x41 && 'a' == 0x61 && '!' == 0x21
 #  define HOST_CHARSET HOST_CHARSET_ASCII
 #else
 # if '\n' == 0x15 && ' ' == 0x40 && '0' == 0xF0 \
    && 'A' == 0xC1 && 'a' == 0x81 && '!' == 0x5A
 #  define HOST_CHARSET HOST_CHARSET_EBCDIC
 # else
 #  define HOST_CHARSET HOST_CHARSET_UNKNOWN
 # endif
 #endif
in include/safe-ctype.h, does that mean we only support EBCDIC if -funsigned-char
and otherwise fail to build gcc?  Because with -fsigned-char, '0' is -0x10
rather than 0xF0, 'A' is -0x3F rather than 0xC1 and 'a' is -0x7F rather than
0x81.
And another thing, if HOST_CHARSET == HOST_CHARSET_EBCDIC, how does the libcpp/lex.c
static const cppchar_t utf8_signifier = 0xC0;
...
      if (*buffer->cur >= utf8_signifier)
        {
          if (_cpp_valid_utf8 (pfile, &buffer->cur, buffer->rlimit, 1 + !first,
                               state, &s))
            return true;
        }
work?  Because in UTF-EBCDIC, >= 0xC0 isn't the right test for start of
multi-byte character, it is more complicated and seems _cpp_valid_utf8
assumes UTF-8 as the host charset.

2021-10-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102615
	* g++.dg/cpp23/charlit-encoding1.C: New testcase for C++23 P2316R2.