[lldb] Rename SetResultIsInternal to SetSuppressPersistentResult (NFC)

Rename `SetResultIsInternal` and `GetResultIsInternal` to `SetSuppressPersistentResult`
and `GetSuppressPersistentResult` respectively.  Also rename `m_result_is_internal`.

This matches the naming in the SB API.

A separate change calls `SetSuppressPersistentResult`, where the name
`SetResultIsInternal` doesn't quite fit.

Differential Revision: https://reviews.llvm.org/D144042