Handle `VarDebugInfoContents::Composite` in `GotocCtx::find_debug_info`

[zhassan-aws]

The Composite variant of the VarDebugInfoContents enum was added in rust-lang/rust#102570, and is concerned with the scalar replacement of aggregates (SROA) MIR optimization that is only enabled with --mir-opt-level=3 or higher.

In #1927, we handled this new variant by considering it to not match the debug info for any Local, which would result in the callers (codegen_var_base_name and codegen_var_name) producing a name that excludes the variable name from the debug info. We need to figure out if this is OK to do, e.g. by experimenting with one of the unit tests in rust-lang/rust#102570.