[clang] Rework the printing of attributes (PR #87281)

[Kim Gräsman via cfe-commits]

kimgr wrote:

A note from left field: I think this PR broke the IWYU test suite. We use `TemplateDecl::print` + some postprocessing to generate template forward-declarations. We're seeing this diff now:
```diff
-template <typename T> class FinalTemplate;
+template <typename T> class  FinalTemplate;
```
So a spurious extra space.

We would be grateful if this _didn't_ make the 18.x branches, because we've already released an 18.x-compatible version of IWYU, and it would be a shame if that release had a failing test suite when built against later 18.x Clang.

I'll work around this on our latest mainline.

https://github.com/llvm/llvm-project/pull/87281