[clang][dataflow] Eliminate `RecordValue::getChild()`. (#65586)

We want to eliminate the `RecordStorageLocation` from `RecordValue` and,
ultimately, eliminate `RecordValue` entirely (see the discussion linked
in the
`RecordValue` class comment). This is one step in that direction.

To eliminate `RecordValue::getChild()`, we also eliminate the last
remaining
caller, namely the `getFieldValue(const RecordValue *, ...)` overload.
Calls
to this overload have been rewritten to use the
`getFieldValue(const RecordStorageLocation *, ...)` overload. Note that
this
also makes the code slightly simpler in many cases.