[lldb][NFCI] Merge implementations of ObjectFileMachO::GetMinimumOSVersion and...

[lldb][NFCI] Merge implementations of ObjectFileMachO::GetMinimumOSVersion and ObjectFileMachO::GetSDKVersion

These functions do the exact same thing (even if they look slightly
different). I yanked the common implementation, cleaned it up, and
shoved it into its own function.

Differential Revision: https://reviews.llvm.org/D151120