MC: remove unnecessary enumeration prefix

This is an enum class, and will be appropriately prefixed, making the encoding
type prefix redundant.  No change to any uses as the use of this was not yet
introduced.
CodeGen: split out the Win64Exception emitter

Move the Windows unwind information emitter into a separate header.  This is not
related to DWARF based emission.  NFC.
CodeGen: indicate Windows unwind data format

The structures for Windows unwinding are shared across multiple platforms.
Indicate the encoding to be used for the particular target.  Use this to switch
the unwind emitter instantiated by the AsmPrinter.