[clangd] don't add inlay hint for dependent type in structured binding

[v1nh1shungry]

Summary

Currently clangd will display useless inlay hint for dependent type in
structured binding, e.g.

template <class T>
void foobar(T arg) {
  auto [a/*: <dependent type>*/, b/*: <dependent type>*/] = arg;
}

[nridge]

Thanks!

A future enhancement to consider could be, in the following testcase:

template <typename T, typename U>
struct Pair {
  T t;
  U u;
};

template <class T, class U>
void foobar(Pair<T, U> arg) {
  auto [a, b] = arg;
}

to get the hints to be T and U. (Today they are <dependent type>, so the patch is still an improvement in this case.)

[v1nh1shungry]

Thank you for the review!

clang-tools-extra/clangd/InlayHints.cpp

public:
   bool VisitVarDecl(VarDecl *D) {
     // Do not show hints for the aggregate in a structured binding,
     // but show hints for the individual bindings.
     if (auto *DD = dyn_cast<DecompositionDecl>(D)) {
       for (auto *Binding : DD->bindings()) {
         // For structured bindings, print canonical types. This is important
         // because for bindings that use the tuple_element protocol, the
         // non-canonical types would be "tuple_element<I, A>::type".
-        if (auto Type = Binding->getType(); !Type.isNull())
+        if (auto Type = Binding->getType();
+            !Type.isNull() && !Type->isDependentType())
           addTypeHint(Binding->getLocation(), Type.getCanonicalType(),
                       /*Prefix=*/": ");
       }
       return true;
     }
 
     if (auto *AT = D->getType()->getContainedAutoType()) {
       if (AT->isDeduced() && !D->getType()->isDependentType()) {

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

TEST(TypeHints, DependentType) {
   assertTypeHints(R"cpp(
     template <typename T>
     void foo(T arg) {
       // The hint would just be "auto" and we can't do any better.
       auto var1 = arg.method();
       // FIXME: It would be nice to show "T" as the hint.
       auto $var2[[var2]] = arg;
     }
+
+    template <typename T>
+    void bar(T arg) {
+      auto [a, b] = arg;
+    }
   )cpp");
 }
 
 TEST(TypeHints, LongTypeName) {
   assertTypeHints(R"cpp(
     template <typename, typename, typename>
     struct A {};
     struct MultipleWords {};