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

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

LG! There is definitely a cost to changing the diagnostic message as @steakhal pointed out but LLVM in general has the philosophy not being afraid of breaking users to do the right thing. Hopefully, as this is one of the less noisy checkers, the fallout would not be too bad.

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