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

[David Tarditi via cfe-commits]

dtarditi wrote:

@steakhal   Yes, the term garbage has a negative connotation when used as an adjective in English.  By a little snarky, the user meant that the message could be seen as a little overly critical.  The user suggested using more neutral terminology.

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