[llvm] [ci] Add Emscripten workflow (PR #151928)

[Vassil Vassilev via llvm-commits]

vgvassilev wrote:

> > This looks like a niche configuration that should be covered by a buildbot, not pre-commit CI.
> 
> Agreed. Using Mac/Windows runners for almost two hours per PR I'm pretty sure is untenable. Configurations like this are also canonically tested postcommit.

@andrurogerz, does idt really take 2 hours? My understanding is that we will run it on the header files that are changed by the corresponding PR.

https://github.com/llvm/llvm-project/pull/151928