[clang] [Clang] Don't trigger `-Wweak-vtables` on an explicit instantiation declaration (PR #195189)

[Corentin Jabot via cfe-commits]

================
@@ -89,6 +89,19 @@ void uses_templ() {
   TemplVirt<long> l;
 }
 
+namespace pr195110 {
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cor3ntin wrote:

```suggestion
namespace GH195110 {
```

https://github.com/llvm/llvm-project/pull/195189