Core: Make member functions const/static

readability-make-member-function-const finds lots of member functions
that could be made const. This change just picks getter functions that
really should be const.

readability-convert-member-functions-to-static finds non-static member
functions which do not access this. This change turns most of them
into static ones, but leaves some non static to keep the class API
consistent.

Change-Id: I004d6f7fab4375f9902f940f29b4a4f561fc7604
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/vcsmanager.cpp

@@ -79,7 +79,7 @@ public:
         QString topLevel;
     };
 
-    Utils::optional<VcsInfo> findInCache(const QString &dir)
+    Utils::optional<VcsInfo> findInCache(const QString &dir) const
     {
         QTC_ASSERT(QDir(dir).isAbsolute(), return Utils::nullopt);
         QTC_ASSERT(!dir.endsWith(QLatin1Char('/')), return Utils::nullopt);