[CodeCompletion] Strengthen null check to assert NFC

This function is only called with a non-null type, which is the only
reasonable way to call it anyway.

Author: benlangmuir

lib/IDE/CodeCompletion.cpp

@@ -3843,8 +3843,7 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
     if (hasFuncIntroducer || hasVarIntroducer || isKeywordSpecified("override"))
       return;
 
-    if (!CurrTy)
-      return;
+    assert(CurrTy);
     const auto *CD = dyn_cast_or_null<ClassDecl>(CurrTy->getAnyNominal());
     if (!CD)
       return;