Add optimization for generator bodies that don't introduce new members (#1013)

If a generator object literal doesn't add any object members, we
can simply use the parent in its place.

Co-authored-by: Kushal Pisavadia <kushi.p@gmail.com>

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

@@ -0,0 +1,46 @@
+// should only result in one trace in the output; generator object literals that don't add any
+// members should be optimized away.
+res1 {
+  prop = trace("hello")
+}
+
+res2 = (res1) {
+  when (false) {
+    prop2 = 2
+  }
+}
+
+res3 = (res1) {
+  for (_ in List()) {
+    "hello"
+  }
+}
+
+res4 = (res1) {
+  ...List()
+}
+
+res5 = (res1) {
+  when (true) {
+  } else {
+    prop3 = 3
+  }
+}
+
+res6 = (Dynamic) {
+  when (true) {}
+}
+
+res7 = (Mapping) {
+  when (true) {}
+}
+
+class Person {}
+
+res8 = (Person) {
+  when (true) {}
+}
+
+res9 = (Listing) {
+  when (true) {}
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/generatorNoMembers.err

@@ -0,0 +1,20 @@
+res1 {
+  prop = "hello"
+}
+res2 {
+  prop = "hello"
+}
+res3 {
+  prop = "hello"
+}
+res4 {
+  prop = "hello"
+}
+res5 {
+  prop = "hello"
+}
+res6 {}
+res7 {}
+res8 {}
+res9 {}
+pkl: TRACE: "hello" = "hello" (file:///$snippetsDir/input/generators/generatorNoMembers.pkl)