CppTools: Give early warning if kit has no compilers

We keep getting bug reports from users who forgot to set a compiler in
their kit and then get confused by (misleading) warnings from the code
model.
Improve the situation by detecting the condition that a C/C++ project's
current kit does not have a C/C++ compiler and showing a warning for
that in the issues pane.

Task-number: QTCREATORBUG-23247
Change-Id: I10164e85ad595f3a386340e7813d1f3e40fbecb5
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cpptools/cppprojectinfogenerator.h

@@ -54,6 +54,8 @@ private:
 private:
     const QFutureInterface<void> m_futureInterface;
     const ProjectExplorer::ProjectUpdateInfo &m_projectUpdateInfo;
+    bool m_cToolchainMissing = false;
+    bool m_cxxToolchainMissing = false;
 };
 } // namespace Internal
 } // namespace CppTools