CustomToolChain: Allow MSVC output parser on non-Windows systems

To do that, move the existing enumeration-based system to a
string base. MSVC/Windows and Custom/Unix shared the same
enumeration value (3), so we must provide an operating system
aware upgrade path.

Task-number: QTCREATORBUG-16247
Change-Id: I8beeeabc09119fc501933a7287f2f982195363a2
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/clangparser.h

@@ -40,6 +40,8 @@ public:
     ClangParser();
     void stdError(const QString &line);
 
+    static Core::Id id();
+
 private:
     QRegularExpression m_commandRegExp;
     QRegularExpression m_inLineRegExp;