[WebAssembly] Add Wasm SjLj option support for clang

This adds support for Wasm SjLj in clang. Also this sets the new
`-mllvm -wasm-enable-eh` option for Wasm EH.

Note there is a little unfortunate inconsistency there: Wasm EH is
enabled by a clang option `-fwasm-exceptions`, which sets
`-mllvm -wasm-enable-eh` in the backend options. It also sets
`-exception-model=wasm` but this is done in the common code.

Wasm SjLj doesn't have a clang-level option like `-fwasm-exceptions`.
`-fwasm-exceptions` was added because each exception model has its
corresponding `-f***-exceptions`, but I'm not sure if adding a new
option like `-fwasm-sjlj` or something is a good idea.

So the current plan is Emscripten sets `-mllvm -wasm-enable-sjlj` if
Wasm SJLj is enabled in its settings.js, as it does for Emscripten
EH/SjLj (it sets `-mllvm -enable-emscripten-cxx-exceptions` for
Emscripten EH and `-mllvm -enable-emscripten-sjlj` for Emscripten SjLj).
And setting this enables the exception handling feature, and also sets
`-exception-model=wasm`, but this time this is not done in the common
code so we do it ourselves.

Also note that other exception models have 1-to-1 correspondance with
their `-f***-exceptions` flag and their `-exception-model=***` flag, but
because we use `-exception-model=wasm` also for Wasm SjLj while
`-fwasm-exceptions` still means Wasm EH, there is also a little
inconsistency there, but I think it is manageable.

Also this adds various error checking and tests.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D108582

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