[llvm-pdbutil] Fix register enum field dumping/parsing (#82299)
This fixes a bug where parsing PDBs with usages of register enums were
asserting.
The main problem is that printing out the code view register enums are
taken care of here:
https://github.com/nikitalita/llvm-project/blob/e4888a92402f53000a3a5e79d3792c034fc2f343/llvm/lib/ObjectYAML/CodeViewYAMLSymbols.cpp#L152
Which requires a COFF::header in the IO context for the machine type,
which we didn't have when dumping a pdb or parsing a yaml file. So, we
make a fake one with the machine type.