[clangd] Fix unittests in TargetDeclTest bucket (#89630)

This PR fixes the build errors for one of the `clangd` unit tests bucket
similar to the following:

```
.../clang-tools-extra/clangd/unittests/FindTargetTests.cpp:430:29: error: passing no argument for the '...' parameter of a variadic macro is a C++20 extension [-Werror,-Wc++20-extensions]
  430 |   EXPECT_DECLS("AutoTypeLoc");
      |                             ^
.../clang-tools-extra/clangd/unittests/FindTargetTests.cpp:98:9: note: macro 'EXPECT_DECLS' defined here
   98 | #define EXPECT_DECLS(NodeType, ...)                                            \
      |         ^
```

This happens when using a build compiler with #84520. The fix is to
include commas to compensate for empty vararg macro arguments in a few
instances.