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/builtinindexingsupport.cpp

@@ -158,7 +158,8 @@ void indexFindErrors(QFutureInterface<void> &future, const ParseParams params)
         // Parse the file as precisely as possible
         BuiltinEditorDocumentParser parser(file);
         parser.setReleaseSourceAndAST(false);
-        parser.update({CppModelManager::instance()->workingCopy(), nullptr, false});
+        parser.update({CppModelManager::instance()->workingCopy(), nullptr,
+                       Language::Cxx, false});
         CPlusPlus::Document::Ptr document = parser.document();
         QTC_ASSERT(document, return);