Clang: Provide the language option to the backend

Provide the language option (e.g. "-x c++-header") when registering a
translation unit for the editor.

Task-number: QTCREATORBUG-14787
Change-Id: Ie06f9fdab302f1b21ba72cdb65b6aabf9f7bc04c
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

tests/unit/unittest/sourcerangetest.cpp

@@ -82,6 +82,7 @@ protected:
     ClangBackEnd::TranslationUnits translationUnits{projects, unsavedFiles};
     TranslationUnit translationUnit{Utf8StringLiteral(TESTDATA_DIR"/diagnostic_source_range.cpp"),
                                     projectPart,
+                                    Utf8StringVector(),
                                     translationUnits};
     DiagnosticSet diagnosticSet{translationUnit.diagnostics()};
     Diagnostic diagnostic{diagnosticSet.front()};