MC: make DWARF and Windows unwinding handling more similar

Rename member variables and functions for the MCStreamer for DWARF-like
unwinding management.  Rename the Windows ones as well and make the naming and
handling similar across the two.  No functional change intended.
MC: make helper function be more const-correct

Introduce const-ness on parameters, they are used as read-only and should not be
modified.  NFC.
MC: make MCWin64EHInstruction a POD-like struct

This is the first of a number of changes designed to generalise
MCWin64EHInstruction to support different target architectures.  An ordered set
(vector) of these instructions is saved per frame to permit the emission of
information for Windows NT style unwinding.  The only bit of information which
is actually target specific here is the Opcode for the unwinding bytecode.  The
remainder of the information is simply generic information that is relevant to
the Windows NT unwinding model.

Remove the accessors for the fields, making them const and public instead.  Sink
the knowledge of the alias'ed name into the single source and sink a single-use
check method into the use.