[clang] [analyzer] Update the undefined assignment checker diagnostics to not use the term 'garbage' (PR #126596)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM, I agree that "uninitialized" is a better phrasing -- it's shorter and explicitly names the "normal" reason for getting `UndefinedVal`s.

> I think the right path is to issue a more precise error message for out-of-bounds reads. As you point, we could do that by including the bounds checker as part of core checkers. If that checker is too noisy for that, we could create a more limited version that covers the cases that are making it to this checker.

Previously the array bound checker was very noisy, but now its "noise level" is comparable to e.g. `core.NullDereference`, so there is no blocking reason that keeps it out of `core`.

https://github.com/llvm/llvm-project/pull/126596