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/cppprojectpartchooser.h

@@ -25,6 +25,7 @@
 
 #pragma once
 
+#include "cpplanguage.h"
 #include "projectpart.h"
 
 #include <functional>
@@ -52,6 +53,7 @@ public:
                             const ProjectPart::Ptr &manuallySetProjectPart,
                             bool stickToPreviousProjectPart,
                             const ProjectExplorer::Project *activeProject,
+                            Language languagePreference,
                             bool projectHasChanged) const;
 
 private: