[clang] [clang] Disable missing definition warning on pure virtual functions (PR #74510)

[Mariya Podchishchaeva via cfe-commits]

================
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wundefined-func-template %s
+// expected-no-diagnostics
+
+namespace PR74016 {
----------------
Fznamznon wrote:

I think we normally call them GH when a bug was filed as a GitHub issue.
```suggestion
namespace GH74016 {
```

https://github.com/llvm/llvm-project/pull/74510