[clangd] Test case demonstrating variable template issue

[nridge]

Summary

This is not meant to be committed, it's meant to demonstrate an
issue with using RecursiveASTVisitor with TestTU on code that contains a
top-level variable template.

The variable template declaration is never visited, for reasons I don't
fully understand.

[nridge]

Please see further details in this clangd-dev post.

[nridge]

Fixed by D89098.

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

const NamedDecl &findUnqualifiedDecl(ParsedAST &AST, llvm::StringRef Name) {
   return findDecl(AST, [Name](const NamedDecl &ND) {
     if (auto *ID = ND.getIdentifier())
       if (ID->getName() == Name)
         return true;
     return false;
   });
 }
 
+class TestVisitor : public RecursiveASTVisitor<TestVisitor> {
+public:
+  bool VisitVarTemplateDecl(VarTemplateDecl *VTD) {
+    seenVarTemplateDecl = true;
+    return true;
+  }
+
+  bool seenVarTemplateDecl = false;
+};
+
+TEST(TestTU, VariableTemplate) {
+  TestTU TU = TestTU::withCode("template <typename T> int x;");
+  auto AST = TU.build();
+  EXPECT_TRUE(AST.getDiagnostics().empty());
+  TestVisitor Visitor;
+  Visitor.TraverseAST(AST.getASTContext());
+  EXPECT_TRUE(Visitor.seenVarTemplateDecl);
+}
+
 } // namespace clangd
 } // namespace clang