Simplify and correct variable assignments

- Remove asgnMaybeAppend, it was only used to indicate
  that the asignment needs a setDefault. But really, all
  types of variable assignments need setDefault if they're
  self-referential.
- Remove local_append/local_set_default because there was
  no implementation for them written in product_config.rbc
  anyways.
- Correct productConfigVariable.emitDefined using the global
  variables instead of the product config variables.
- Emit setDefaults for all types of assignments if they're
  self referential.

Bug: 222737841
Test: go test
Change-Id: I06a0f90f16d5900049d473281e6d5ef5e93e67da

mk2rbc/node.go

@@ -184,10 +184,9 @@ type assignmentFlavor int
 
 const (
 	// Assignment flavors
-	asgnSet         assignmentFlavor = iota // := or =
-	asgnMaybeSet    assignmentFlavor = iota // ?= and variable may be unset
-	asgnAppend      assignmentFlavor = iota // += and variable has been set before
-	asgnMaybeAppend assignmentFlavor = iota // += and variable may be unset
+	asgnSet      assignmentFlavor = iota // := or =
+	asgnMaybeSet assignmentFlavor = iota // ?=
+	asgnAppend   assignmentFlavor = iota // +=
 )
 
 type assignmentNode struct {
@@ -215,6 +214,20 @@ func (asgn *assignmentNode) emit(gctx *generationContext) {
 	}
 }
 
+func (asgn *assignmentNode) isSelfReferential() bool {
+	if asgn.flavor == asgnAppend {
+		return true
+	}
+	isSelfReferential := false
+	asgn.value.transform(func(expr starlarkExpr) starlarkExpr {
+		if ref, ok := expr.(*variableRefExpr); ok && ref.ref.name() == asgn.lhs.name() {
+			isSelfReferential = true
+		}
+		return nil
+	})
+	return isSelfReferential
+}
+
 type exprNode struct {
 	expr starlarkExpr
 }