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/cppcompletionassist.cpp

@@ -2122,7 +2122,10 @@ void CppCompletionAssistInterface::getCppSpecifics() const
     m_gotCppSpecifics = true;
 
     if (m_parser) {
-        m_parser->update({CppTools::CppModelManager::instance()->workingCopy(), nullptr, false});
+        m_parser->update({CppTools::CppModelManager::instance()->workingCopy(),
+                          nullptr,
+                          Language::Cxx,
+                          false});
         m_snapshot = m_parser->snapshot();
         m_headerPaths = m_parser->headerPaths();
     }