Utils: Add std::expected implementation

Adds a std::expected implementation that is compatible with >= C++11.

FilePath::fileContents and FilePath::writeFileContents as well as
FilePath::copyFile are changed to return std::expected.

A couple of macros have been added to aid in using the expected types.

An auto test was added showing how to use the library.

Change-Id: Ibe3aecfc1029a0cf13b45bf5184ff03a04a2393b
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/android/androidbuildapkstep.cpp

@@ -680,7 +680,9 @@ static bool copyFileIfNewer(const FilePath &sourceFilePath,
 
     if (!destinationFilePath.parentDir().ensureWritableDir())
         return false;
-    return sourceFilePath.copyFile(destinationFilePath);
+    expected_str<void> result = sourceFilePath.copyFile(destinationFilePath);
+    QTC_ASSERT_EXPECTED(result, return false);
+    return true;
 }
 
 void AndroidBuildApkStep::doRun()