[lldb] Fix missing characters when autocompleting LLDB commands in REPL

Summary:

When tabbing to complete LLDB commands in REPL, characters would at best be
missing but at worst cause the REPL to crash due to out of range string access.
This patch appends the command character to the completion results to fulfill
the assumption that all matches are prefixed by the request's cursor argument
prefix.

Bug report for the Swift REPL
https://bugs.swift.org/browse/SR-12867

Reviewers: teemperor

Reviewed By: teemperor

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D82835