Add as_expression_list to reflection APIs

Style note: I'm currently trying to follow the pattern of choosing ".get_Y" vs. ".as_Y" names as follows...

"get": X::is_Y / X::get_Y pairs
   - when Y is conceptually "NOT all of" the X
   - for example, expression_statement::get_expression because even though the expression is most of the statement the expression-statement contains that extra `;` and is at a different conceptual layer

"as": X::is_Y / X::as_Y pairs
   - when Y is conceptually "all of" the X
   - for example, statement::as_expression_statement because it's really still the whole statement
   - with the idea that this could legitimately become an actual `as` operator

I may not be 100% consistent on that right now, but I think the current API is pretty close to that

Author: hsutter

regression-tests/test-results/version

@@ -1,4 +1,4 @@
 
-cppfront compiler v0.8.2 Build A427:1507
+cppfront compiler v0.8.2 Build A427:1609
 SPDX-License-Identifier  Apache-2.0 WITH LLVM-exception
 Copyright (c) 2022-2024  Herb Sutter

source/build.info

@@ -1 +1 @@
-"A427:1507"
+"A427:1609"

source/parse.h

@@ -161,6 +161,9 @@ struct primary_expression_node
     auto is_expression_list() const
         -> bool;
 
+    auto get_expression_list()
+        -> expression_list_node*;
+
     auto get_expression_list() const
         -> expression_list_node const*;
 
@@ -286,6 +289,9 @@ struct prefix_expression_node
     auto is_expression_list() const
         -> bool;
 
+    auto get_expression_list()
+        -> expression_list_node*;
+
     auto get_expression_list() const
         -> expression_list_node const*;
 
@@ -413,6 +419,15 @@ struct binary_expression_node
         return terms.empty() && expr->is_expression_list();
     }
 
+    auto get_expression_list()
+        -> expression_list_node*
+    {
+        if (is_expression_list()) {
+            return expr->get_expression_list();
+        }
+        return {};
+    }
+
     auto get_expression_list() const
         -> expression_list_node const*
     {
@@ -628,6 +643,15 @@ struct expression_node
         return expr->is_expression_list();
     }
 
+    auto get_expression_list()
+        -> expression_list_node *
+    {
+        if (is_expression_list()) {
+            return expr->get_expression_list();
+        }
+        return {};
+    }
+
     auto get_expression_list() const
         -> expression_list_node const*
     {
@@ -836,6 +860,15 @@ auto primary_expression_node::is_expression_list() const
     return expr.index() == expression_list;
 }
 
+auto primary_expression_node::get_expression_list()
+    -> expression_list_node *
+{
+    if (is_expression_list()) {
+        return std::get<expression_list>(expr).get();
+    }
+    return {};
+}
+
 auto primary_expression_node::get_expression_list() const
     -> expression_list_node const*
 {
@@ -1035,6 +1068,15 @@ struct postfix_expression_node
         return ops.empty() && expr->is_expression_list();
     }
 
+    auto get_expression_list() 
+        -> expression_list_node *
+    {
+        if (is_expression_list()) {
+            return expr->get_expression_list();
+        }
+        return {};
+    }
+
     auto get_expression_list() const
         -> expression_list_node const*
     {
@@ -1128,6 +1170,15 @@ auto prefix_expression_node::is_expression_list() const
     return ops.empty() && expr->is_expression_list();
 }
 
+auto prefix_expression_node::get_expression_list() 
+    -> expression_list_node *
+{
+    if (is_expression_list()) {
+        return expr->get_expression_list();
+    }
+    return {};
+}
+
 auto prefix_expression_node::get_expression_list() const
     -> expression_list_node const*
 {
@@ -1658,6 +1709,15 @@ struct is_as_expression_node
         return ops.empty() && expr->is_expression_list();
     }
 
+    auto get_expression_list() 
+        -> expression_list_node *
+    {
+        if (is_expression_list()) {
+            return expr->get_expression_list();
+        }
+        return {};
+    }
+
     auto get_expression_list() const
         -> expression_list_node const*
     {