Add support for Wasm EH

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Adds codegen and DRuntime support for WebAssembly exception handling.

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Some IR comparisons for reference

C++/Clang: https://godbolt.org/z/heMqqGvbb
D/LDC (this PR): https://godbolt.org/z/vMYWYvPE5

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@kinke

Maybe @rainers?

It works, but maybe one of you has a better idea on how to organize the modifications to ‎gen/trycatchfinally.cpp‎.

Wasm EH follows MSVC style catchpad/catchswitch/catchret & cleanpad/cleanupret. However there is only one catchswitch per catchpad, after which an Itanium-landingpad-style comparison chain needs to be emitted to determine the correct catch block (if any).

Right now I'm treating it more like MSVC, and just dumping everything in emitCatchBodies...though this requires exposing some TryCatchFinallyScope internals...