ld: Handle extended-length data structures in PDB types

A few fixes to minor issues I've discovered in my PDB patches.

* If sizes or offsets are greater than 0x8000, they get encoded as
extended values in the same way as for enum values - e.g. a LF_ULONG
.short followed by a .long.

* I've managed to coax MSVC to produce another type, LF_VFTABLE, which
is seen when dealing with COM. I don't think LLVM emits this. Note that
we can't just implement everything in Microsoft's header files, as most
of it is obsolete.

* Fixes a stupid bug in the test program, where I was adding an index to
a size. The index was hard-coded to 0, so this didn't cause any actual
issues.