Avoid non-trivially shadowing string.toString()

Prepare libraries for a possible deprecation warning on shadowing
string.toString().

These instanceof classes were using this.(Type).method() to call
supertype methods, but super.method() is clearer and equivalent
for instanceof supertypes.

Author: ginsbach

shared/regex/codeql/regex/nfa/ExponentialBackTracking.qll

@@ -95,7 +95,7 @@ module Make<RegexTreeViewSig TreeImpl> {
       )
     }
 
-    string toString() { result = this.(InfiniteRepetitionQuantifier).toString() }
+    string toString() { result = super.toString() }
   }
 
   /**

shared/regex/codeql/regex/nfa/NfaUtils.qll

@@ -104,11 +104,11 @@ module Make<RegexTreeViewSig TreeImpl> {
   private class RegexpCharacterConstant instanceof RegExpConstant {
     RegexpCharacterConstant() { this.isCharacter() }
 
-    string toString() { result = this.(RegExpConstant).toString() }
+    string toString() { result = super.toString() }
 
-    RegExpTerm getRootTerm() { result = this.(RegExpConstant).getRootTerm() }
+    RegExpTerm getRootTerm() { result = super.getRootTerm() }
 
-    string getValue() { result = this.(RegExpConstant).getValue() }
+    string getValue() { result = super.getValue() }
   }
 
   /**
@@ -578,11 +578,11 @@ module Make<RegexTreeViewSig TreeImpl> {
       )
     }
 
-    string toString() { result = this.(RegExpTerm).toString() }
+    string toString() { result = super.toString() }
 
-    RegExpTerm getAChild() { result = this.(RegExpTerm).getChild(_) }
+    RegExpTerm getAChild() { result = super.getChild(_) }
 
-    RegExpTerm getChild(int i) { result = this.(RegExpTerm).getChild(i) }
+    RegExpTerm getChild(int i) { result = super.getChild(i) }
   }
 
   /**
@@ -601,11 +601,11 @@ module Make<RegexTreeViewSig TreeImpl> {
       )
     }
 
-    string toString() { result = this.(RegExpTerm).toString() }
+    string toString() { result = super.toString() }
 
-    RegExpTerm getAChild() { result = this.(RegExpTerm).getAChild() }
+    RegExpTerm getAChild() { result = super.getAChild() }
 
-    RegExpTerm getChild(int i) { result = this.(RegExpTerm).getChild(i) }
+    RegExpTerm getChild(int i) { result = super.getChild(i) }
   }
 
   /**
@@ -621,11 +621,11 @@ module Make<RegexTreeViewSig TreeImpl> {
       )
     }
 
-    string toString() { result = this.(RegExpTerm).toString() }
+    string toString() { result = super.toString() }
 
-    RegExpTerm getAChild() { result = this.(RegExpTerm).getAChild() }
+    RegExpTerm getAChild() { result = super.getAChild() }
 
-    RegExpTerm getChild(int i) { result = this.(RegExpTerm).getChild(i) }
+    RegExpTerm getChild(int i) { result = super.getChild(i) }
   }
 
   /**