[DebugInfo] Fix printing of DW_LNS_set_isa

The Isa register is a uint8_t, but at least on Windows this is
internally an unsigned char, which meant that prior to this patch it got
formatted as an ASCII character, rather than a decimal number. This
patch fixes this by casting it to a uint64_t before printing. I did it
this way instead of using a uint8_t formatter because a) it is simpler,
and b) it allows us to change the internal type of Isa in the future
without this code breaking.

I also took the opportunity to test the printing of the other standard
opcodes.

Reviewed by: probinson

Differential Revision: https://reviews.llvm.org/D71274