[clangd] Fix -Wunused-variable warning (NFC)

GCC warning:
```
/llvm-project/clang-tools-extra/clangd/InlayHints.cpp: In member function ‘bool clang::clangd::InlayHintVisitor::VisitVarDecl(clang::VarDecl*)’:
/llvm-project/clang-tools-extra/clangd/InlayHints.cpp:81:15: warning: unused variable ‘AT’ [-Wunused-variable]
   81 |     if (auto *AT = D->getType()->getContainedAutoType()) {
      |               ^~

```