ProjectExplorer: Fix Clang toolchain setup on Windows

Clang toolchain in gcc mode requires mingw sysroot and target
in order to be used properly on Windows.
Requires Qt >= 5.12 to work properly (it has the required mkspec).

Change-Id: I4e5a734c699ac98740c0d50560aa7b69751ae58c
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/toolchain.cpp

@@ -362,6 +362,11 @@ QList<Task> ToolChain::validateKit(const Kit *) const
     return QList<Task>();
 }
 
+QString ToolChain::sysRoot() const
+{
+    return QString();
+}
+
 /*!
     \class ProjectExplorer::ToolChainFactory
     \brief The ToolChainFactory class creates tool chains from settings or