Eagerly check function arguments when called from inside iterable (#778)

This mitigates an issue where lazy mappings and listings widen an existing bug.

This is a follow-up to https://github.com/apple/pkl/pull/752.

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorNestedReference.pkl

@@ -31,3 +31,27 @@ res2 {
     }.birds
   }
 }
+
+res3 {
+  for (key, _ in Map("hello-there", 5)) {
+    ...myself(new Listing {
+      new Listing {
+        key
+      }
+    })
+  }
+}
+
+res4 {
+  for (key, _ in Map("hello-there", 5)) {
+    ...myself2.apply(new Listing {
+      new Listing {
+        key
+      }
+    })
+  }
+}
+
+function myself(l: Listing<Listing<String>>) = l
+
+local myself2 = (l: Listing<Listing<String>>) -> l

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorNestedReference.pcf

@@ -8,3 +8,13 @@ res2 {
     age = 1
   }
 }
+res3 {
+  new {
+    "hello-there"
+  }
+}
+res4 {
+  new {
+    "hello-there"
+  }
+}