C#: Simplify library instantiations.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowReachability.qll

@@ -26,17 +26,7 @@ private module ControlFlowInput implements InputSig<Location, ControlFlowNode, B
 
   class Expr = CS::Expr;
 
-  class SourceVariable = Ssa::SourceVariable;
-
-  class SsaDefinition = Ssa::Definition;
-
-  class SsaExplicitWrite extends SsaDefinition instanceof Ssa::ExplicitDefinition {
-    Expr getValue() { result = super.getADefinition().getSource() }
-  }
-
-  class SsaPhiDefinition = Ssa::PhiNode;
-
-  class SsaUncertainWrite = Ssa::UncertainDefinition;
+  import Ssa
 
   class GuardValue = Guards::GuardValue;
 

csharp/ql/lib/semmle/code/csharp/controlflow/Guards.qll

@@ -191,23 +191,7 @@ private module GuardsImpl = SharedGuards::Make<Location, Cfg, GuardsInput>;
 class GuardValue = GuardsImpl::GuardValue;
 
 private module LogicInput implements GuardsImpl::LogicInputSig {
-  class SsaDefinition extends Ssa::Definition {
-    Expr getARead() { super.getARead() = result }
-  }
-
-  class SsaExplicitWrite extends SsaDefinition instanceof Ssa::ExplicitDefinition {
-    Expr getValue() { result = super.getADefinition().getSource() }
-  }
-
-  class SsaPhiDefinition extends SsaDefinition instanceof Ssa::PhiNode {
-    predicate hasInputFromBlock(SsaDefinition inp, BasicBlock bb) {
-      super.hasInputFromBlock(inp, bb)
-    }
-  }
-
-  class SsaParameterInit extends SsaDefinition instanceof Ssa::ParameterDefinition {
-    Parameter getParameter() { result = super.getParameter() }
-  }
+  import Ssa
 
   predicate additionalNullCheck(GuardsImpl::PreGuard guard, GuardValue val, Expr e, boolean isNull) {
     // Comparison with a non-`null` value, for example `x?.Length > 0`

csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll

@@ -250,9 +250,9 @@ private predicate defReaches(Ssa::Definition def, ControlFlowNode cfn) {
 }
 
 private module NullnessConfig implements ControlFlowReachability::ConfigSig {
-  predicate source(ControlFlowNode node, Ssa::Definition def) { defMaybeNull(def, node, _, _) }
+  predicate source(ControlFlowNode node, SsaDefinition def) { defMaybeNull(def, node, _, _) }
 
-  predicate sink(ControlFlowNode node, Ssa::Definition def) {
+  predicate sink(ControlFlowNode node, SsaDefinition def) {
     exists(Dereference d |
       dereferenceAt(node, def, d) and
       not d instanceof NonNullExpr