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

[Donát Nagy via cfe-commits]

NagyDonat wrote:

Yes, obviously "garbage" has negative connotations, but we want to report an _error_, so we need to have some negative connotations. It's pointless to start an [euphemism treadmill](https://en.wiktionary.org/wiki/euphemism_treadmill) because it just introduces churn without actually going forward.

I'd suggest pushing back against this request, citing the precedent that "garbage collector" is also an established term.

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