Address post commit review comments on revision 366727.
Addresses number of comment made on D64652 after commiting:
- Reorders function decls in the TargetLoweringObjectFileXCOFF class.
- Fix comment in MCSectionXCOFF to include description of external reference csects.
- Convert several llvm_unreachables to report_fatal_error
- Convert several dyn_casts to casts as they are expected not to fail.
- Avoid copying DataLayout object.