[clang-tools-extra] [clang-doc] Improve performance by adding a short circuit (PR #96809)

[Paul Kirth via cfe-commits]

================
@@ -12,16 +12,21 @@
 #include "clang/AST/Comment.h"
 #include "clang/Index/USRGeneration.h"
 #include "llvm/ADT/StringExtras.h"
-#include "llvm/Support/Error.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/Support/Mutex.h"
 
 namespace clang {
 namespace doc {
 
+static llvm::StringSet USRVisited;
----------------
ilovepi wrote:

```suggestion
static llvm::StringSet<> USRVisited;
```
There's a warning about type deduction (`-Wctad-maybe-unsupported`)

https://github.com/llvm/llvm-project/pull/96809