[lldb] Set default object format to `MachO` in `ObjectFileMachO` (#142704)

# The Change

This patch sets the **default** object format of `ObjectFileMachO` to be
`MachO` (instead of what currently ends up to be `ELF`, see below). This
should be **the correct thing to do**, because the code before the line
of change has already verified the Mach-O header.

The existing logic:
* In `ObjectFileMachO`, the object format is unassigned by default. So
it's `UnknownObjectFormat` (see
[code](https://github.com/llvm/llvm-project/blob/54d544b83141dc0b20727673f68793728ed54793/llvm/lib/TargetParser/Triple.cpp#L1024)).
* The code then looks at load commands like `LC_VERSION_MIN_*`
([code](https://github.com/llvm/llvm-project/blob/54d544b83141dc0b20727673f68793728ed54793/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp#L5180-L5217))
and `LC_BUILD_VERSION`
([code](https://github.com/llvm/llvm-project/blob/54d544b83141dc0b20727673f68793728ed54793/lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp#L5231-L5252))
and assign the Triple's OS and Environment if they exist.
* If the above sets the Triple's OS to macOS, then the object format
defaults to `MachO`; otherwise it is `ELF`
([code](https://github.com/llvm/llvm-project/blob/54d544b83141dc0b20727673f68793728ed54793/llvm/lib/TargetParser/Triple.cpp#L936-L937))

# Impact

For **production usage** where Mach-O files have the said load commands
(which is
[expected](https://www.google.com/search?q=Are+mach-o+files+expected+to+have+the+LC_BUILD_VERSION+load+command%3F)),
this patch won't change anything.
* **Important note**: It's not clear if there are legitimate production
use cases where the Mach-O files don't have said load commands. If there
is, the exiting code think they are `ELF`. This patch changes it to
`MachO`. This is considered a fix for such files.

For **unit tests**, this patch will simplify the yaml data by not
requiring the said load commands.

# Test

See PR.