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.h

@@ -55,6 +55,7 @@ public:
 public:
     Core::Id clangDiagnosticConfigId() const;
     void setClangDiagnosticConfigId(const Core::Id &configId);
+    void resetClangDiagnosticConfigId();
     const ClangDiagnosticConfig clangDiagnosticConfig() const;
 
     ClangDiagnosticConfigs clangCustomDiagnosticConfigs() const;