Fix #12711 FN unusedVariable for std::*stringstream with constructor use (#6600)

Author: chrchr-github

cfg/std.cfg

@@ -8998,6 +8998,12 @@ initializer list (7) string& replace (const_iterator i1, const_iterator i2, init
       <check>std::filesystem::filesystem_error</check>
       <check>std::bad_variant_access</check>
       <check>std::span</check>
+      <check>std::stringstream</check>
+      <check>std::wstringstream</check>
+      <check>std::istringstream</check>
+      <check>std::wistringstream</check>
+      <check>std::ostringstream</check>
+      <check>std::wostringstream</check>
     </unusedvar>
     <operatorEqVarError>
       <suppress>std::mutex</suppress>

test/cfg/std.cpp

@@ -5128,3 +5128,13 @@ std::size_t variableScope_cstr(const char* p) {
     }
     return std::strlen(p);
 }
+
+void unusedvar_stringstream(const char* p)
+{
+    // cppcheck-suppress unreadVariable
+    std::istringstream istr(p);
+    // cppcheck-suppress unreadVariable
+    std::ostringstream ostr(p);
+    // cppcheck-suppress unreadVariable
+    std::stringstream sstr(p);
+}