Add semantic::Statement::Block() for getting the owning BlockStatement

Required special casing the ElseStatement, as this isn't actually owned by a BlockStatement.

Change-Id: Ic33c207598b838a12b865a7694e596b2629c9208
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/46443
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/ast/block_statement.h

@@ -65,6 +65,9 @@ class BlockStatement : public Castable<BlockStatement, Statement> {
   /// @returns the ending iterator
   StatementList::const_iterator end() const { return statements_.end(); }
 
+  /// @returns the statement list
+  const StatementList& statements() const { return statements_; }
+
   /// Clones this node and all transitive child nodes using the `CloneContext`
   /// `ctx`.
   /// @param ctx the clone context