[Swift+WASM] Allow Wasm object format.

[ddunbar]

• This currently does nothing more than adopt the ELF conventions, but for the
  Wasm object file format.

[ddunbar]

Since it seems the most modern wasm triple also sets the wasm object file format (#20684), this is necessary when using that triple.

I don't really know the extent to which any of this makes sense, although my impression is that the wasm format is largely following ELF, so I imagine they aren't absolutely horrible defaults to start with exploring.

[ddunbar]

@swift-ci please smoke test

[compnerd]

You're going to have to look at SwiftRT-ELF.cpp and make equivalences for the WASM object format. Additionally, do you know what linkers are supported? I think that you want to enable the ODR COMDAT semantics for WASM as I believe that lld is linker there and that works properly unlike gold or bfd.

[ddunbar]

@compnerd I believe that linking support is under active development, and as you say is only in lld. Thanks for the pointer, I'll take a look!

[jfbastien]

You'll probably want to look at https://github.com/WebAssembly/tool-conventions

[DougGregor]

I don't know much about Wasm, but this looks reasonable as a variant of ELF

[MaxDesiatov]

@ddunbar any updates on this? hope this could be merged soon

[ddunbar]

See https://bugs.swift.org/browse/SR-9307 for higher level status. I was wanting to get the Clang change landed first although there is probably no reason not to merge this one

[ddunbar]

@swift-ci please smoke test

[ddunbar]

Going ahead and merging this to unblock further poking... this will evolve and eventually get tests once enough other pieces are in that an end-to-end test can work.