`FindStorageArgForInitializerInSpecific` is missing a `SpecificConstant`
case, and crashes due to the function not expecting `SpecificConstant`.
`SpecificConstant` should be handled by this function, as it derives its
expression category from the instruction it wraps.
Consider this example:
```carbon
interface I {}
interface J {}
constraint C {
extend require impls I & J;
}
```
Calls, such as `I & J`, are initializing expressions. `extend require
impls` wraps the call in a `SpecificConstant`. This code crashes when
asking for its storage, such as during formatting for `--dump-sem-ir`.
This commit adds a case for `SpecificConstant`, deferring to its wrapped
instruction when transitive lookup is allowed, and reports no storage
otherwise.
Assisted-by: Antigravity with Claude Opus 5.5
Christopher Di Bella
04f456dddd