Clang: Fix crash when removing diagnostic config

1. Create some Qt Console Application from the wizard
  2. Open Tools > Options > C++ > Code Model
    2.1 Click "Manage..." and create a custom configuration "custom" and
      finish with OK.
    2.2 Set "custom" as diagnostic config.
  3. Analyze > "Clang-Tidy and Clazy..."
    3.1 Use "Custom Settings"
    3.2 Click "Manage..." and remove "custom" ==> Crash

The crash happened because the code model used an invalid diagnostic
config id. The invalid id came from the settings, which were not
correctly upated.

Fixes: QTCREATORBUG-21273
Change-Id: I5242f7c92e121eec8558fa7923139bb3d759c676
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/cppcodemodelsettings.cpp

@@ -195,6 +195,11 @@ void CppCodeModelSettings::setClangDiagnosticConfigId(const Core::Id &configId)
     m_clangDiagnosticConfigId = configId;
 }
 
+void CppCodeModelSettings::resetClangDiagnosticConfigId()
+{
+    m_clangDiagnosticConfigId = initialClangDiagnosticConfigId();
+}
+
 const ClangDiagnosticConfig CppCodeModelSettings::clangDiagnosticConfig() const
 {
     const ClangDiagnosticConfigsModel configsModel(m_clangCustomDiagnosticConfigs);