C++: make MemberFunction::getTypeOfThis() return PointerType

The dbscheme technically allows for any Type, but in practice it will
always be a PointerType, so this should make it easier for users to
understand.

Author: nickrolfe

cpp/ql/src/semmle/code/cpp/MemberFunction.qll

@@ -72,10 +72,10 @@ class MemberFunction extends Function {
   }
 
   /**
-   * Gets the type of the `this` parameter associated with this member function, if any. Typically,
-   * this will be a `PointerType`, possibly with `const` and/or `volatile` qualifiers.
+   * Gets the type of the `this` parameter associated with this member function, if any. The type
+   * may have `const` and/or `volatile` qualifiers, matching the function declaration.
    */
-  Type getTypeOfThis() {
+  PointerType getTypeOfThis() {
     member_function_this_type(underlyingElement(this), unresolveElement(result))
   }
 }