[lldb][NFC] Refactor remaining completion logic to use CompletionRequests

This patch moves the remaining completion functions from the
old completion API (that used several variables) to just
passing a single CompletionRequest.

This is for the most part a simple change as we just replace
the old arguments with a single CompletionRequest argument.

There are a few places where I had to create new CompletionRequests
in the called functions as CompletionRequests itself are immutable
and don't expose their internal match list anymore. This means that
if a function wanted to change the CompletionRequest or directly
access the result list, we need to work around this by creating
a new CompletionRequest and a temporary match/description list.

Preparation work for rdar://53769355

llvm-svn: 369000