[lldb] Refactor deduction of the instance variable's name (NFC)

Move responsibility of providing the instance variable name (`this`, `self`) from
`TypeSystem` to `Language`.

`Language` the natural place for this, but also has downstream benefits. Some languages
have multiple `TypeSystem` implementations (ex Swift), and by placing this logic in the
`Language`, redundancy is avoided.

This change relies on the tests from D145348 and D146320.

Differential Revision: https://reviews.llvm.org/D146548