fix Issue 19959 - Eponymous template FQN's re-state the template name

[rainers]

2 caveats:

• json output changes, too. Does it affect tools that might read and interpret the name?
• explicitly specified eponymous symbols get removed, too, e.g. in https://github.com/rainers/dmd/blob/a6bbb7216f76d48622b7e1cd75b4ba8cb649159c/test/fail_compilation/ice9540.d

[dlang-bot]

Thanks for your pull request, @rainers!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19959	enhancement	Eponymous template FQN's re-state the template name

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#10846"

[RazvanN7]

Since the logic is duplicated, I suggest moving the check in a function that receives a ScopeDsymbol (or a template function that receives a template instance or a template declaration) so that we save some lines of code.