Fix #14720 FP assertWithSideEffect (const function call QDir::exists() in assert) (cppcheck-opensource#8526)

Co-authored-by: chrchr-github <noreply@github.com>

Author: chrchr-github

cfg/qt.cfg

@@ -2428,6 +2428,7 @@
   <!-- bool QDir::exists() const -->
   <function name="QDir::exists">
     <noreturn>false</noreturn>
+    <pure/>
     <returnValue type="bool"/>
     <use-retval/>
     <leak-ignore/>
@@ -2440,6 +2441,7 @@
   <!-- bool QFile::exists() const -->
   <function name="QFile::exists">
     <noreturn>false</noreturn>
+    <pure/>
     <returnValue type="bool"/>
     <use-retval/>
     <leak-ignore/>

test/cfg/qt.cpp

@@ -76,6 +76,11 @@ void ignoredReturnValue_QDir(const QString& dirname)
     dir.entryList();
 }
 
+void assertWithSideEffect_QDir_exists(const char *path) { // #14720
+    QDir dir(path);
+    assert(dir.exists());
+    (void)dir;
+}
 
 void unusedVariable_QTransform()
 {