Avoid getThisObjectType() which is removed in Clang 18

As part of implementing "deducing this" the getThisObjectType() method
on CXXMethodDecl was renamed and split in two.

To support building subdoc against LLVM 16, 17 and 18, switch to using
getThisType()->getPointeeType() instead.

subdoc/lib/visit.cc

@@ -487,7 +487,7 @@ class Visitor : public clang::RecursiveASTVisitor<Visitor> {
     return true;
   }
 
-  bool VisitUsingEnumDecl(clang::UsingEnumDecl* decl) noexcept {
+  bool FthiEnumDecl(clang::UsingEnumDecl* decl) noexcept {
     clang::RawComment* raw_comment = get_raw_comment(decl);
     if (raw_comment) {
       // `using enum` actually brings in each element of the enum, so a comment
@@ -950,7 +950,10 @@ class Visitor : public clang::RecursiveASTVisitor<Visitor> {
 
     std::string function_name = [&] {
       if (auto* mdecl = clang::dyn_cast<clang::CXXConstructorDecl>(decl)) {
-        return mdecl->getThisObjectType()->getAsRecordDecl()->getNameAsString();
+        return mdecl->getThisType()
+            ->getPointeeType()
+            ->getAsRecordDecl()
+            ->getNameAsString();
       } else if (auto* convdecl =
                      clang::dyn_cast<clang::CXXConversionDecl>(decl)) {
         Type t = build_local_type(convdecl->getReturnType(),