[StackIR] Support source maps and DWARF with StackIR

[kripken]

Helping #6509, this fixes debugging support for StackIR, which makes it more
possible to use StackIR in more places.

The fix is basically just to pass around some more state, and then to call the
parent with "please write debug info" at the correct times, mirroring the
similar calls in BinaryenIRWriter.

The relevant Emscripten tests pass, and the source map test modified
here produces identical output in StackIR and non-StackIR modes.

Remove --new-wat-parser in the test here - @tlively we don't need that
anymore do we?

[tlively]

Code LGTM, so LGTM overall if the test changes make sense.

[tlively]

Is this expected and accounted for?

[kripken]

I believe so, yes. StackIR saves a few bytes here and there, which allows encoded offsets to be a bit shorter in some cases. Any increase in size (in this or any other field) would have been very suspicious, however. A large decrease would also have been odd.

I did also run llvm-dwarfdump --validate on the output and it is equally valid as before this PR (which is, 99% valid but for one known pre-existing minor issue).

[kripken]

@tlively what about this question?

Remove --new-wat-parser in the test here - @tlively we don't need that anymore do we?

(if we actually don't then I can simplify the test a little more I think)

[tlively]

Oh yes, sorry. --new-wat-parser can be removed.

[kripken]

Thanks, then I simplified the test now to have autogenerated CHECKs, since the checks are identical between StackIR and BinaryenIR (and without two parsers, there is just one output for BinaryenIR).