feat(3p/nix): Add function to allocate a Value in traceable memory

Backported from:
b3e5eea4a9
fcd048a526

Intentionally skipped because we have not backported the JSON changes:
9f46f54de4

Did not apply changes ni primops.cc, because those look suspect and
are also based on something that we don't have in our tree.

Change-Id: I837787ce9f2c90267bc39fce15177980d209d4e9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1253
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/value.hh

@@ -247,4 +247,6 @@ typedef std::map<Symbol, Value*, std::less<Symbol>,
                  traceable_allocator<std::pair<const Symbol, Value*>>>
     ValueMap;
 
+std::shared_ptr<Value*> allocRootValue(Value* v);
+
 }  // namespace nix