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/eval.cc

@@ -5,10 +5,13 @@
 #include <cstring>
 #include <fstream>
 #include <iostream>
+#include <memory>
 #include <new>
 #include <optional>
 #include <variant>
 
+#define GC_INCLUDE_NEW
+
 #include <absl/strings/match.h>
 #include <gc/gc.h>
 #include <gc/gc_cpp.h>
@@ -40,6 +43,10 @@ static char* dupString(const char* s) {
   return t;
 }
 
+std::shared_ptr<Value*> allocRootValue(Value* v) {
+  return std::allocate_shared<Value*>(traceable_allocator<Value*>(), v);
+}
+
 static void printValue(std::ostream& str, std::set<const Value*>& active,
                        const Value& v) {
   checkInterrupt();