C#: Refactor recursive patterns implementation

- Extract names of properties in a propery match, using the `exprorstmt_name` relation.
- Simplify extraction of properties by not distinguishing between top-level patterns
  and nested patterns.
- Introduce `PatternExpr` to capture patterns in `is` expressions, `case` statements,
  and `switch` expression arms.
- Generalize `IsTypeExpr`, `IsPatternExpr`, `IsRecursivePatternExpr`, and `IsConstantExpr`
  to just `IsExpr` with a member predicate `PatternExpr getPattern()`.
- Generalize `TypeCase`, `RecursivePatternCase`, and `ConstCase` to just `CaseStmt` with
  a member predicate `PatternExpr getPattern()`.
- Introduce classes `Switch` and `Case` as base classes of switch statements/expressions
  and case statements/switch expression arms, respectively.
- Simplify CFG logic using the generalized classes.
- Generalize guards library to cover `switch` expressions tests.
- Generalize data flow library to cover `switch` expression assignments.

csharp/ql/src/Dead Code/DeadStoreOfLocal.ql

@@ -87,9 +87,7 @@ class RelevantDefinition extends AssignableDefinition {
         lvde = any(ForeachStmt fs).getVariableDeclExpr()
       )
     or
-    this instanceof AssignableDefinitions::IsPatternDefinition
-    or
-    this instanceof AssignableDefinitions::TypeCasePatternDefinition
+    this instanceof AssignableDefinitions::PatternDefinition
   }
 
   /** Holds if this assignment may be live. */