Fix the violation handler noexcept work

The test needs to set an exception mask on cerr for streambuf exceptions to work.
Also modify the noexceptness of the correct version of the violation handler,
i.e. the nonattr one, not the legacy C++2a one.

Author: villevoutilainen

gcc/testsuite/g++.dg/contracts/cpp26/contract-violation-noexcept.C

@@ -1,8 +1,7 @@
 // test that the default contract violation handler can't throw
 // { dg-do run }
-// { dg-options "-std=c++2a -fcontracts -fcontract-continuation-mode=on -fcontracts-nonattr " }
+// { dg-options "-std=c++2a -fcontracts -fcontract-evaluation-semantic=observe -fcontracts-nonattr " }
 
-#include <experimental/contract>
 #include <iostream>
 #include <exception>
 #include <cstdlib>
@@ -75,14 +74,17 @@ public:
 
 typedef  fail_buf<std::streambuf>   fail_streambuf;
 
-
 // Test that there is an active exception when we reach the terminate handler.
 void my_term()
 {
   try { throw; }
-  catch(...) { std::exit(0); }
+  catch(const underflow_error&) { std::exit(0); }
+  catch(const overflow_error&) { std::exit(0); }
+  catch(const positioning_error&) { std::exit(0); }
 }
 
+
+
 void f(int x) pre(x >= 0)
 {
   try{
@@ -97,6 +99,7 @@ int main()
   std::set_terminate (my_term);
   fail_streambuf buf;
   std::cerr.rdbuf(&buf);
+  std::cerr.exceptions(std::ios::badbit | std::ios::failbit | std::ios::eofbit);
   try
   {
       f(-42);

libstdc++-v3/src/experimental/contract.cc

@@ -29,7 +29,7 @@
 #endif
 
 __attribute__ ((weak)) void
-handle_contract_violation (const std::experimental::contract_violation &violation) noexcept
+handle_contract_violation (const std::experimental::contract_violation &violation)
 {
 #if _GLIBCXX_HOSTED && _GLIBCXX_VERBOSE
   bool level_default_p = violation.assertion_level() == "default";

libstdc++-v3/src/experimental/contract26.cc

@@ -30,7 +30,7 @@
 #endif
 
 __attribute__ ((weak)) void
-handle_contract_violation (const std::contracts::contract_violation &violation)
+handle_contract_violation (const std::contracts::contract_violation &violation) noexcept
 {
 #if _GLIBCXX_HOSTED && _GLIBCXX_VERBOSE