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/baseeditordocumentprocessor.cpp

@@ -25,8 +25,11 @@
 
 #include "baseeditordocumentprocessor.h"
 
+#include "cppcodemodelsettings.h"
+#include "cpplanguage.h"
 #include "cppmodelmanager.h"
 #include "cpptoolsbridge.h"
+#include "cpptoolsreuse.h"
 #include "editordocumenthandle.h"
 
 #include <projectexplorer/session.h>
@@ -55,8 +58,13 @@ BaseEditorDocumentProcessor::~BaseEditorDocumentProcessor()
 
 void BaseEditorDocumentProcessor::run(bool hasActiveProjectChanged)
 {
+    const Language languagePreference = codeModelSettings()->interpretAmbigiousHeadersAsCHeaders()
+            ? Language::C
+            : Language::Cxx;
+
     runImpl({CppModelManager::instance()->workingCopy(),
              ProjectExplorer::SessionManager::startupProject(),
+             languagePreference,
              hasActiveProjectChanged});
 }