Use template syntax to declare rules for variadic functions

This allows writing

```cpp
template <typename... Args>
int f1(int a0, int a1, Args... args) {
  return fcntl(a0, a1, args...);
}
```

Which becomes `f1: int fcntl(int, int, ...)`

Author: lucic71

cpp2rust/converter/mapper.cpp

@@ -804,6 +804,12 @@ std::string ToString(const clang::NamedDecl *decl) {
     }
     os << ToString(func_decl->getParamDecl(i)->getType());
   }
+  if (func_decl->isVariadic()) {
+    if (func_decl->getNumParams()) {
+      os << ", ";
+    }
+    os << "...";
+  }
   os << ')';
 
   if (const auto *method_decl =

cpp2rust/cpp_rule_preprocessor.cpp

@@ -349,7 +349,10 @@ class Callback : public clang::ast_matchers::MatchFinder::MatchCallback {
   createTemplateArguments(clang::TemplateDecl *decl,
                           llvm::SmallVectorImpl<clang::TemplateArgument> &out) {
     for (clang::NamedDecl *param : *decl->getTemplateParameters()) {
-      if (llvm::isa<clang::TemplateTypeParmDecl>(param)) {
+      if (param->isTemplateParameterPack()) {
+        out.emplace_back(
+            clang::TemplateArgument::CreatePackCopy(sema_->Context, {}));
+      } else if (llvm::isa<clang::TemplateTypeParmDecl>(param)) {
         clang::RecordDecl *rdecl = createRecordDecl(param->getName());
         clang::QualType type =
             sema_->Context.getTagType(clang::ElaboratedTypeKeyword::None,
@@ -409,7 +412,14 @@ class Callback : public clang::ast_matchers::MatchFinder::MatchCallback {
                          clang::OverloadCandidateSet &candidates) {
     clang::LookupResult decls(*sema_, name, loc_,
                               clang::Sema::LookupOrdinaryName);
-    sema_->LookupQualifiedName(decls, sema_->getStdNamespace());
+    if (clang::NamespaceDecl *std_ns = sema_->getStdNamespace()) {
+      sema_->LookupQualifiedName(decls, std_ns);
+    }
+    if (decls.empty()) {
+      decls.clear();
+      sema_->LookupQualifiedName(decls,
+                                 sema_->Context.getTranslationUnitDecl());
+    }
     for (auto *ndecl : decls) {
       if (auto *candidate = createCandidate(ndecl, callArgs, explicitTArgs)) {
         sema_->AddOverloadCandidate(
@@ -573,6 +583,7 @@ class Callback : public clang::ast_matchers::MatchFinder::MatchCallback {
       cxxConstructorNameLookup(rule->getReturnType(), callArgs, candidates);
       break;
     case LookupKind::ADL:
+      regularNameLookup(callArgs, &explicitTArgs, name, candidates);
       adlLookup(callArgs, name, candidates);
       break;
     }