[clang] [NFC][Clang] Adopt `TrailingObjects` convenience API in MacroArgs (PR #139635)

[Kazu Hirata via cfe-commits]

kazutakahirata wrote:

@jurahul Thanks for the pointer in the description.  So, in this particular case, we end up invoking the following?

```
  FirstTrailingType *getTrailingObjects() {
    static_assert(sizeof...(TrailingTys) == 1,
                  "Can use non-templated getTrailingObjects() only when there "
                  "is a single trailing type");
    return getTrailingObjects<FirstTrailingType>();
  }
```


https://github.com/llvm/llvm-project/pull/139635