MC: rename MCW64UnwindInfo to MCWinFrameInfo

This structure contains information related to the call frame used to generate
unwinding information.  Rename this to reflect the future use to represent the
shared state between various architectures for WinCFI information.
MC: remove use of unnecessary variable

Due to the fact that the windows unwinding has the concept of chained frames, we
maintain a current frame info pointer that is adjusted on any push and pop of a
unwinding context.  This just removes an unnecessary variable that was used to
mirror the DWARF unwinding code.