[clang][NFC] fix name lookup for llvm::json::Value in SymbolGraphSerializer (#94511)

This code uses namespaces `llvm` and `llvm::json`. However, we have both
`llvm::Value` and `llvm::json::Value`. Whenever any of the headers
declare or include `llvm::Value`, the lookup becomes ambiguous.

Fixing this by qualifying the `Value` type.

Author: yuxuanchen1997

clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp

@@ -31,7 +31,6 @@
 using namespace clang;
 using namespace clang::extractapi;
 using namespace llvm;
-using namespace llvm::json;
 
 namespace {
 
@@ -1036,9 +1035,9 @@ void SymbolGraphSerializer::serializeGraphToStream(
     ExtendedModule &&EM) {
   Object Root = serializeGraph(ModuleName, std::move(EM));
   if (Options.Compact)
-    OS << formatv("{0}", Value(std::move(Root))) << "\n";
+    OS << formatv("{0}", json::Value(std::move(Root))) << "\n";
   else
-    OS << formatv("{0:2}", Value(std::move(Root))) << "\n";
+    OS << formatv("{0:2}", json::Value(std::move(Root))) << "\n";
 }
 
 void SymbolGraphSerializer::serializeMainSymbolGraph(