[AST] Tighten up type of `InitContext`
Switch from promising a DeclContext to a
PatternBindingInitializer.
This has a couple of benefits:
- It eliminates a few places where we were force
`cast`'ing to PatternBindingInitializer.
- It improves the clarity of what's being stored,
it's not whatever the parent context of the
initializer is, it's specifically the
PatternBindingInitializer context if it exists.
