SBAddress currently *may* have an Address object or it may not.

If it has an Address object, it is assumed to be Valid.
Change SBAddress to always have an Address object and check
whether it is valid or not in those case.

This is fixing a subtle problem where we ended up with
a SBAddress with an Address of LLDB_INVALID_ADDRESS could
run through a copy constructor and turn into an SBAddress
with no Address object being backed (because it wasn't
distinguishing between invalid-Address versus no-Address.)

The cost of an Address object is not high and this will be
an easy mistake for someone else to make; I'm fixing
SBAddress so it doesn't come up again.
<rdar://problem/18069407> 

llvm-svn: 221002