MC: make WinEH opcode an opaque value

This makes the opcode an opaque value (unsigned int) rather than the
enumeration.  This permits the use of target specific operands.

Split out the generic type into a MCWinEH header and add a supporting
MCWin64EH::Instruction to abstract out the selection of the opcode and
construction of the actual instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213221 91177308-0d34-0410-b5e6-96231b3b80d8