[llvm-objdump][COFF] Correctly decode `UOP_Epilog` opcodes

At least `ntdll` is using the undocumented version 2 unwind info, and opcode 6, which is already defined as `UOP_Epilog`.
Using `llvm-objdump --unwind` with `ntdll` would previously result in unreachable assertions because this code was missing from `getNumUsedSlots` and `getUnwindCodeTypeName`.
The slots of these codes comes from https://github.com/dotnet/runtime/blob/57bfe474518ab5b7cfe6bf7424a79ce3af9d6657/src/coreclr/inc/win64unwind.h#L51-L52 which I would assume is a good authoritative source.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D107655