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

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

Fixes clangd/clangd#121

Differential Revision: https://reviews.llvm.org/D116921

Author: hokein

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

@@ -96,9 +96,7 @@ bool ExpandAutoType::prepare(const Selection& Inputs) {
   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;

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

@@ -71,7 +71,8 @@ TEST_F(ExpandAutoTypeTest, Test) {
       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};");