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