ClangFormat: Fix GCC8 warnings

* Missing return (false positive. All enum values are covered).
* Unused static function.

Amends commits 7f0589ef59 and
40fcc728b2.

Change-Id: I0e5c4ae09739a7ca9edbbbbe3f081d8a732245e7
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangformat/clangformatbaseindenter.cpp

@@ -188,8 +188,6 @@ QByteArray dummyTextForContext(CharacterContext context, bool closingBraceBlock)
     }
 
     switch (context) {
-    case CharacterContext::Unknown:
-        QTC_ASSERT(false, return "";);
     case CharacterContext::AfterComma:
         return "a,";
     case CharacterContext::NewStatement:
@@ -197,6 +195,9 @@ QByteArray dummyTextForContext(CharacterContext context, bool closingBraceBlock)
     case CharacterContext::Continuation:
     case CharacterContext::LastAfterComma:
         return "& a &";
+    case CharacterContext::Unknown:
+    default:
+        QTC_ASSERT(false, return "";);
     }
 }