Java: Add StmtParent as superclass of SwitchExpr

Database type `@stmtparent` already includes `@switchexpr`, this commit merely
changes the class SwitchExpr to also accordingly extend StmtParent.

Author: Marcono1234

java/ql/src/semmle/code/java/Expr.qll

@@ -1280,7 +1280,7 @@ class ConditionalExpr extends Expr, @conditionalexpr {
 /**
  * A `switch` expression.
  */
-class SwitchExpr extends Expr, @switchexpr {
+class SwitchExpr extends Expr, StmtParent, @switchexpr {
   /** Gets an immediate child statement of this `switch` expression. */
   Stmt getAStmt() { result.getParent() = this }