Fix '23_containers/span/at.cc' for '-fno-exceptions' configurations
Added in recent commit
1fa85dcf656e2f2c7e483c9ed3c2680bf7db6858
"libstdc++: Add std::span::at for C++26 (P2821R5)", the test case already
does use '#if __cpp_exceptions', but failed to correspondingly guard the
'dg-warning' directives, resulting in:
FAIL: 23_containers/span/at.cc -std=gnu++26 (test for warnings, line 15)
FAIL: 23_containers/span/at.cc -std=gnu++26 (test for warnings, line 26)
PASS: 23_containers/span/at.cc -std=gnu++26 (test for excess errors)
PASS: 23_containers/span/at.cc -std=gnu++26 execution test
libstdc++-v3/
* testsuite/23_containers/span/at.cc: Fix for '-fno-exceptions'
configurations.