[clang] [analyzer] Prettify checker registration and unittest code (PR #147797)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

@Szelethus I implemented the changes that you suggested in person.

@steakhal FYI I reduced the scope of this change:
- I turned `DocsUri` into an optional argument (with a default value) instead of outright removing it from the `public` overloads.
- I removed `addMockChecker` because @Szelethus suggested that we should not include test-specific utilities in the non-test code.

https://github.com/llvm/llvm-project/pull/147797