CppTools: Allow interpreting ambiguous headers as C headers

...instead of C++ headers.

For the Clang Code Model this results in using "-x c-header" instead of
"-x c++-header".

This introduces a new option in Options > C++ > "Code Model" to
configure this.

Change-Id: I8a0ce8fa6155f5ef58743ebc7f1d0b500fbf6599
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/cpptools/cppcodemodelsettingspage.h

@@ -55,13 +55,11 @@ public:
     void applyToSettings() const;
 
 private:
+    void setupGeneralWidgets();
     void setupClangCodeModelWidgets();
-    void setupPchCheckBox() const;
-    void setupSkipIndexingFilesWidgets();
 
+    bool applyGeneralWidgetsToSettings() const;
     bool applyClangCodeModelWidgetsToSettings() const;
-    bool applyPchCheckBoxToSettings() const;
-    bool applySkipIndexingFilesWidgets() const;
 
 private:
     Ui::CppCodeModelSettingsPage *m_ui;