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/clangcodemodel/clangeditordocumentparser.cpp

@@ -43,6 +43,7 @@ void ClangEditorDocumentParser::updateImpl(const QFutureInterface<void> &,
                                               configuration(),
                                               state_,
                                               updateParams.activeProject,
+                                              updateParams.languagePreference,
                                               updateParams.hasActiveProjectChanged);
     setState(state_);
 }