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

[Rahul Joshi via cfe-commits]

jurahul wrote:

I filled in the description now. I am doing similar changes on LLVM/MLIR side (change has been approved).

I added these convenience APIs to TrailingObjects to support the common code patterns of a single trailing type as well as creating an ArrayRef from the trailing objects pointer.

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