Fix and clean up default procedure parameter code for non-constant types

src/entity.cpp

@@ -81,6 +81,7 @@ enum ParameterValueKind {
 
 struct ParameterValue {
 	ParameterValueKind kind;
+	Ast *original_ast_expr;
 	union {
 		ExactValue value;
 		Ast *ast_value;
@@ -126,6 +127,7 @@ struct Entity {
 			i32        field_src_index;
 
 			ParameterValue param_value;
+			Ast *          param_expr;
 
 			String     thread_local_model;
 			Entity *   foreign_library;