gh-104683: Improve consistency and test coverage of argument-clinic __repr__ functions

[AlexWaygood]

Several of the __repr__ functions in Tools/clinic/ have no test coverage at the moment, so this PR started life as an attempt to get some easy coverage improvements. While working on adding coverage for the __repr__ functions, however, I noticed that the reprs for classes in Tools/clinic/ can be pretty inconsistent -- e.g. Function objects describe themselves as clinic.Function objects in their reprs, but Destination objects just describe themselves as Destination objects. Some classes quoted their self.name attribute in their __repr__ methods; others didn't. This PR therefore also improves the consistency of the __repr__ methods in Tools/clinic/.

Coverage prior to this PR:

Name                     Stmts   Miss Branch BrPart  Cover
----------------------------------------------------------
Tools\clinic\clinic.py    2947    135   1236    114    94%
Tools\clinic\cpp.py        102      9     54      2    90%
----------------------------------------------------------
TOTAL                     3049    144   1290    116    94%

Coverage with this PR:

Name                     Stmts   Miss Branch BrPart  Cover
----------------------------------------------------------
Tools\clinic\clinic.py    2948    119   1236    115    94%
Tools\clinic\cpp.py        103      8     54      2    91%
----------------------------------------------------------
TOTAL                     3051    127   1290    117    94%

So, no change to the top-line stat, but a reasonable reduction in uncovered statements.

• Issue: Modernise code in Tools/clinic/ #104683