[clang][Sema] Always clear UndefinedButUsed (#73955)

Before, it was only cleared if there were undefined entities. This is
important for Clang's incremental parsing as used by `clang-repl` that
might receive multiple calls to `Sema.ActOnEndOfTranslationUnit`.

clang/lib/Sema/Sema.cpp

@@ -870,6 +870,7 @@ static void checkUndefinedButUsed(Sema &S) {
   // Collect all the still-undefined entities with internal linkage.
   SmallVector<std::pair<NamedDecl *, SourceLocation>, 16> Undefined;
   S.getUndefinedButUsed(Undefined);
+  S.UndefinedButUsed.clear();
   if (Undefined.empty()) return;
 
   for (const auto &Undef : Undefined) {
@@ -923,8 +924,6 @@ static void checkUndefinedButUsed(Sema &S) {
     if (UseLoc.isValid())
       S.Diag(UseLoc, diag::note_used_here);
   }
-
-  S.UndefinedButUsed.clear();
 }
 
 void Sema::LoadExternalWeakUndeclaredIdentifiers() {