[clang] [clang][AST] Fix printing `TagDecl`s. (PR #69971)

[Richard Smith via cfe-commits]

zygoloid wrote:

It looks like this regressed in https://github.com/llvm/llvm-project/commit/9359b8fe5d944c8355ea49ba949d481fe080cec9 -- prior to that, we did check whether the tag declaration in the group was a definition before deciding whether to include it as part of the group.

https://github.com/llvm/llvm-project/pull/69971