mitigated C4834 Visual Studio and -Wunused-result Clang warnings

```
warning C4834: discarding return value of function with [[nodiscard]] attribute
```

```
error: ignoring return value of function declared with 'nodiscard' attribute [clang-diagnostic-unused-result]
```

Author: firewave

gui/checkthread.cpp

@@ -264,7 +264,7 @@ void CheckThread::runAddonsAndTools(const Settings& settings, const FileSettings
                     }
                     f1.close();
                 }
-                f1.open(QIODevice::WriteOnly | QIODevice::Text);
+                (void)f1.open(QIODevice::WriteOnly | QIODevice::Text); // TODO: check result
                 QTextStream out1(&f1);
                 out1 << chksum;
 

gui/fileviewdialog.cpp

@@ -62,7 +62,7 @@ void FileViewDialog::loadTextFile(const QString &filename, QTextEdit *edit)
         return;
     }
 
-    file.open(QIODevice::ReadOnly | QIODevice::Text);
+    (void)file.open(QIODevice::ReadOnly | QIODevice::Text); // TODO: check result
     if (!file.isReadable()) {
         QString msg(tr("Could not read the file: %1"));
         msg = msg.arg(filename);

gui/mainwindow.cpp

@@ -1717,7 +1717,7 @@ void MainWindow::complianceReport()
     }
 
     QTemporaryFile tempResults;
-    tempResults.open();
+    (void)tempResults.open(); // TODO: check result
     tempResults.close();
 
     mUI->mResults->save(tempResults.fileName(), Report::XMLV2, mCppcheckCfgProductName);

tools/triage/mainwindow.cpp

@@ -112,7 +112,7 @@ void MainWindow::loadFile()
     if (fileName.isEmpty())
         return;
     QFile file(fileName);
-    file.open(QIODevice::ReadOnly | QIODevice::Text);
+    (void)file.open(QIODevice::ReadOnly | QIODevice::Text); // TODO: check result
     QTextStream textStream(&file);
     load(textStream);
 }