Fix source mapping of generated cache wrapper calls

In the SWC transform we're now using the span of the original `"use
cache"` function for the generated cache wrapper call, e.g. so that the
captured timeout error stack can be source mapped correctly.

This also fixes the wrongly placed
`/*#__TURBOPACK_DISABLE_EXPORT_MERGING__*/` marker for the generated
cache wrapper statements.

crates/next-custom-transforms/src/transforms/server_actions.rs

@@ -703,7 +703,7 @@ impl<C: Comments> ServerActions<C> {
                     span: DUMMY_SP,
                     kind: VarDeclKind::Var,
                     decls: vec![VarDeclarator {
-                        span: DUMMY_SP,
+                        span: arrow.span,
                         name: Pat::Ident(cache_ident.clone().into()),
                         init: Some(wrap_cache_expr(
                             Box::new(Expr::Fn(FnExpr {
@@ -841,7 +841,7 @@ impl<C: Comments> ServerActions<C> {
                     span: DUMMY_SP,
                     kind: VarDeclKind::Var,
                     decls: vec![VarDeclarator {
-                        span: DUMMY_SP,
+                        span: function.span,
                         name: Pat::Ident(cache_ident.clone().into()),
                         init: Some(wrap_cache_expr(
                             Box::new(Expr::Fn(FnExpr {