[Support] Ensure complete type DelimitedScope (llvm#127459)

`JSONScopedPrinter` has a `std::unique_ptr<DelimitedScope>` member and
defaulted constructor argument, so it needs a complete type. This
resolves one of the many build errors with C++23 using Clang.

(cherry picked from commit e65d388)

Author: hahnjo

llvm/include/llvm/Support/ScopedPrinter.h

@@ -539,7 +539,13 @@ ScopedPrinter::printHex<support::ulittle16_t>(StringRef Label,
   startLine() << Label << ": " << hex(Value) << "\n";
 }
 
-struct DelimitedScope;
+struct DelimitedScope {
+  DelimitedScope(ScopedPrinter &W) : W(&W) {}
+  DelimitedScope() : W(nullptr) {}
+  virtual ~DelimitedScope() = default;
+  virtual void setPrinter(ScopedPrinter &W) = 0;
+  ScopedPrinter *W;
+};
 
 class JSONScopedPrinter : public ScopedPrinter {
 private:
@@ -838,14 +844,6 @@ class JSONScopedPrinter : public ScopedPrinter {
   }
 };
 
-struct DelimitedScope {
-  DelimitedScope(ScopedPrinter &W) : W(&W) {}
-  DelimitedScope() : W(nullptr) {}
-  virtual ~DelimitedScope() = default;
-  virtual void setPrinter(ScopedPrinter &W) = 0;
-  ScopedPrinter *W;
-};
-
 struct DictScope : DelimitedScope {
   explicit DictScope() = default;
   explicit DictScope(ScopedPrinter &W) : DelimitedScope(W) { W.objectBegin(); }