[clangd] Enable expand-auto for decltype(auto).

[hokein]

Summary

Based on https://reviews.llvm.org/D116919.

Fixes https://github.com/clangd/clangd/issues/121

[thakis]

This broke check-clangd: http://45.33.8.238/linux/64747/step_9.txt (maybe triggered by the revert for D116919 which broke check-clang-tools.)

clang-tools-extra/clangd/refactor/tweaks/ExpandAutoType.cpp

bool isTemplateParam(const SelectionTree::Node *Node) {
   return false;
 }
 
 bool ExpandAutoType::prepare(const Selection& Inputs) {
   CachedLocation = llvm::None;
   if (auto *Node = Inputs.ASTSelection.commonAncestor()) {
     if (auto *TypeNode = Node->ASTNode.get<TypeLoc>()) {
       if (const AutoTypeLoc Result = TypeNode->getAs<AutoTypeLoc>()) {
-        // Code in apply() does handle 'decltype(auto)' yet.
-        if (!Result.getTypePtr()->isDecltypeAuto() &&
-            !isStructuredBindingType(Node) &&
+        if (!isStructuredBindingType(Node) &&
             !isDeducedAsLambda(Node, Result.getBeginLoc()) &&
             !isTemplateParam(Node))
           CachedLocation = Result;
       }
     }
   }
 
   return (bool) CachedLocation;

clang-tools-extra/clangd/unittests/tweaks/ExpandAutoTypeTests.cpp

EXPECT_EQ(apply(R"cpp(au^to x = "test";)cpp"),
             R"cpp(const char * x = "test";)cpp");
 
   EXPECT_EQ(apply("ns::Class * foo() { au^to c = foo(); }"),
             "ns::Class * foo() { ns::Class * c = foo(); }");
   EXPECT_EQ(
       apply("void ns::Func() { au^to x = new ns::Class::Nested{}; }"),
       "void ns::Func() { ns::Class::Nested * x = new ns::Class::Nested{}; }");
 
-  EXPECT_UNAVAILABLE("dec^ltype(au^to) x = 10;");
+  EXPECT_EQ(apply("dec^ltype(auto) x = 10;"), "int x = 10;");
+  EXPECT_EQ(apply("decltype(au^to) x = 10;"), "int x = 10;");
   // expanding types in structured bindings is syntactically invalid.
   EXPECT_UNAVAILABLE("const ^auto &[x,y] = (int[]){1,2};");
 
   // unknown types in a template should not be replaced
   EXPECT_THAT(apply("template <typename T> void x() { ^auto y = T::z(); }"),
               StartsWith("fail: Could not deduce type for 'auto' type"));
 
   ExtraArgs.push_back("-std=c++17");
   EXPECT_UNAVAILABLE("template <au^to X> class Y;");
 }
 
 } // namespace
 } // namespace clangd
 } // namespace clang