[lldb][NFC] Fix style issues with StackID.h (#157483)
Some comments were "suffixed" to member variable declarations; these are
moved to before the variable.
Some constructors and operators were just defaulted and not necessary.
Some comments dividing the class into logical sections, like "//
constructors and destructors", were not applied everywhere. These were
removed. They are used in some parts of LLDB, but are the exception.
An include was not needed.
The operator != can be defined in terms of ==.
