Add a RegisterNumber class to RegisterContextLLDB.h and start using
it in RegisterContext.cpp.

There's a lot of bookkeeping code in RegisterContextLLDB where it has
to convert between different register numbering schemes and it makes
some methods like SavedLocationForRegister very hard to read or
maintain.  Abstract all of the details about different register numbering
systems for a given register into this new class to make it easier
to understand what the method is doing.

Also add register name printing to all of the logging -- that's easy to
get now that I've got an object to represent the register numbers.

There were some gnarly corner cases of this method that I believe
I've translated correctly - initial testing looks good but it's
possible I missed a corner case, especially with architectures which
uses a link-register aka return address register like arm32/arm64.
Basic behavior is correct but there are a lot of corner casese that are
handled in this method ...