[clang] [NFC][Clang] Adopt simplified `getTrailingObjects` in DeclCXX (PR #140078)

[Rahul Joshi via cfe-commits]

jurahul wrote:

> Hi, no objections to the patch, but I am wondering if there is a larger reason behind this effort to simplify all these uses of getTrailingObjects.
> 
> Are you planning on an API change in this area for instance?

Not really. I added simplifications to the TrailingObjects class last week and this is just a series of changes to adopt the simplifications in the codebase. Nothing more beyond such NFC patches planned.

https://github.com/llvm/llvm-project/pull/140078