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

[David Tarditi via cfe-commits]

dtarditi wrote:

@haoNoQ thanks for the explanation! Yes, this makes sense.  I agree that the right long-term approach is that the ArrayBoundChecker issues warnings about out-of-bounds accesses.  In general, I think it is desirable to report a problem about undefined behavior as close to its cause as possible, using a logical explanation where possible.

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