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/customparser.cpp

@@ -140,6 +140,11 @@ void CustomParser::setSettings(const CustomParserSettings &settings)
     m_warning = settings.warning;
 }
 
+Core::Id CustomParser::id()
+{
+    return Core::Id("ProjectExplorer.OutputParser.Custom");
+}
+
 bool CustomParser::hasMatch(const QString &line, CustomParserExpression::CustomParserChannel channel,
                             const CustomParserExpression &expression, Task::TaskType taskType)
 {