[MC] [Win64EH] Avoid producing malformed xdata records

If there's no unwinding opcodes, omit writing the xdata/pdata records.

Previously, this generated truncated xdata records, and llvm-readobj
would error out when trying to print them.

If writing of an xdata record is forced via the .seh_handlerdata
directive, skip it if there's no info to make a sensible unwind
info structure out of, and clearly error out if such info appeared
later in the process.

Differential Revision: https://reviews.llvm.org/D86527