This adds a unique ID to the COFF section uniquing map, similar to the
one we have for ELF. The unique id is not currently exposed via the
assembler because we don't have a use case for it yet. Users generally
create .pdata with the .seh_* family of directives, and the assembler
internally needs to produce .pdata and .xdata sections corresponding to
the code section.
The map to remember which .pdata section to use for a given .text
section is maintained in the MCContext, since this is something the
assembler needs to maintain to implement .seh_* directives. I'm open to
suggestisons for a better home.