[NFC][AIX] Use assert instead of llvm_unreachable

Addresses post-commit comments on https://reviews.llvm.org/D64825. Use
assert instead of llvm_unreachable to check if invalid csect types are being
generated. Use report_fatal_error on unimplemented XCOFF features.

Differential Revision: https://reviews.llvm.org/D64825

llvm-svn: 368720