C#: Reinstate toString for SSA data flow nodes.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/dataflow/internal/SsaImpl.qll

@@ -77,6 +77,10 @@ module Ssa_ = Impl::MakeSsa<SsaInput>;
 
 class Definition = Impl::Definition;
 
+private class SsaDefinitionToStringProxy extends Definition {
+  override string toString() { result = this.(SsaDefinition).toString() }
+}
+
 deprecated class WriteDefinition = Impl::WriteDefinition;
 
 deprecated class UncertainWriteDefinition = Impl::UncertainWriteDefinition;