DWARF support for multiple line tables

[kripken]

Multiple tables appear to be emitted when linking files
together. This fixes our support for that, which did not
update their size properly. This required patching the
YAML emitting code from LLVM in order to measure
the size and then emit it, as that code is apparently
not designed to handle changes in line table
contents.

Other minor fixes:

• Set the flags for our dwarfdump command to emit
  the same as llvm-dwarfdump does with -v -all.
• Add support for a few more opcodes,
  set_discriminator, set_basic_block, fixed_advance_pc,
  set_isa.
• Handle a compile unit without abbreviations in the
  YAML code (again, apparently not something this
  LLVM code was intended to do).
• Handle a compile unit with zero entries in the
  YAML code (ditto).
• Properlty set the AddressSize - we use the
  DWARFContext in a different way than LLVM expects,
  apparently.

With this the great majority of the emscripten test
suite passes with -gforce_dwarf.

My overall impression so from the the YAML code is
that it probably isn't a long-term solution for us. Perhaps
it may end up being scaffolding, that is, we can
replace it with our own code eventually that is based
on it, and remove most of the LLVM code. Before
deciding that we should get everything working first,
and this seems like the quickest path there.