[lldb] Fix that `process signal` completion always returns all signals

`CompletionRequest::AddCompletion` adds the given string as completion of the
current command token. `CompletionRequest::TryCompleteCurrentArg` only adds it
if the current token is a prefix of the given string. We're using
`AddCompletion` for the `process signal` handler which means that `process
signal SIGIN` doesn't get uniquely completed to `process signal SIGINT` as we
unconditionally add all other signals (such as `SIGABRT`) as possible
completions.

By using `TryCompleteCurrentArg` we actually do the proper filtering which will
only add `SIGINT` (as that's the only signal with the prefix 'SIGIN' in the
example above).

Reviewed By: mib

Differential Revision: https://reviews.llvm.org/D105028