[lldb] Fix completion of 'settings set' values

Some time ago, a refactor (1153dc96) broke completion for assigning settings
values (`settings set`). This was most annoying for enum settings, where you'd
have to get the valid enum names separately.

This restores the logic in the post-refactor completion function, as well as
adding a test to catch future regressions.

Differential Revision: https://reviews.llvm.org/D134515