I neglected to add a test when I was writing tests for this, so of
course it broke. This makes it work again and adds a test.

rdar://159459160

The LLVM Style Guide says the following about error and warning messages
[1]:

> [T]o match error message styles commonly produced by other tools,
> start the first sentence with a lowercase letter, and finish the last
> sentence without a period, if it would end in one otherwise.

I often provide this feedback during code review, but we still have a
bunch of places where we have inconsistent error message, which bothers
me as a user. This PR identifies a handful of those places and updates
the messages to be consistent.

[1] https://llvm.org/docs/CodingStandards.html#error-and-warning-messages

Also fixes our Windows on Arm bot.

This fixes issues found in e066f35c6981c720e3a7e5883efc40c861b3b7, which
was later reverted. The problem was with "k" message which was sent with
sync_on_timeout flag set to true, so lldb was waiting for response,
which is currently not being sent by lldb-server. Not waiting for
response at all seems to be not a solution, because on MAC OS X lldb
waits for response from "k" to gracefully kill inferior.

Tests using ObjC do not readily run on Linux.

The use of `-lobjc` resulted in this test failing to build on Linux. The
flag is not necessary, this removes the flag with the expectation it
will fix the test on Linux.

Use the current frame's language to lookup commands provided by language plugins.

This means commands like `language {objc,cplusplus} <command>` can be used directly, without using the `language <lang>` prefix.

For example, when stopped on a C++ frame, `demangle _Z1fv` will run `language cplusplus demangle _Z1fv`.

rdar://149882520

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>

If your arguments or option values are of a type that naturally uses one
of our common completion mechanisms, you will get completion for free.
But if you have your own custom values or if you want to do fancy things
like have `break set -s foo.dylib -n ba<TAB>` only complete on symbols
in foo.dylib, you can use this new mechanism to achieve that.

Among other things, returning an empty string as the repeat command
disables auto-repeat, which can be useful for state-changing commands.

There's one remaining refinement to this setup, which is that for parsed
script commands, it should be possible to change an option value, or add
a new option value that wasn't originally specified, then ask lldb "make
this back into a command string". That would make doing fancy things
with repeat commands easier.

That capability isn't present in the lldb_private side either, however.
So that's for a next iteration.

I haven't added this to the docs on adding commands yet. I wanted to
make sure this was an acceptable approach before I spend the time to do
that.