fix(3p/nix): inherit Expr from gc, make parser state traceable

The parser contained vectors, and the primary parser state, that
were not participating in GC tracing.

Change-Id: Ie198592cd7acffd390e3e2ae9595138b56416838
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1706
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/parser.hh

@@ -5,6 +5,7 @@
 #include <optional>
 #include <variant>
 
+#include <gc/gc.h>
 #include <glog/logging.h>
 
 #include "libexpr/eval.hh"
@@ -59,8 +60,7 @@ void addAttr(ExprAttrs* attrs, AttrPath& attrPath, Expr* e, const Pos& pos);
 
 void addFormal(const Pos& pos, Formals* formals, const Formal& formal);
 
-Expr* stripIndentation(const Pos& pos, SymbolTable& symbols,
-                       std::vector<Expr*>& es);
+Expr* stripIndentation(const Pos& pos, SymbolTable& symbols, VectorExprs& es);
 
 Path resolveExprPath(Path path);