[analyzer] [NFC] Add a helper for getting SVal for 'this' to CXXMemberCall

[NoQ]

What was wrong with getCXXThisVal()?

[george.karpenkov]

OK

clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h

public:
   }
 
   unsigned getNumArgs() const override {
     if (const CallExpr *CE = getOriginExpr())
       return CE->getNumArgs();
     return 0;
   }
 
+  SVal getThisSVal() const {
+    return getSVal(getCXXThisExpr());
+  }
+
   const Expr *getArgExpr(unsigned Index) const override {
     return getOriginExpr()->getArg(Index);
   }
 
   const Expr *getCXXThisExpr() const override;
 
   RuntimeDefinition getRuntimeDefinition() const override;