[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.