Adapt to using SPIR-V support as an external library

[thewilsonator]

For use with https://github.com/KhronosGroup/SPIRV-LLVM-Translator now that I finally managed to get around to wrangling CMake, after breaking my backend upgrading LLVM.

I'll add a release for OSX soon for testing, but this is something we can potentially do for LDC releases when we ship with LLVM7.

[thewilsonator]

Fixed semicolons, fixed windows (good god I hate it).

[thewilsonator]

I'll do a release of LLVM sometime this week.

[thewilsonator]

Release done, heres hoping that travis config works.

[thewilsonator]

Finally it passes.

[thewilsonator]

Thanks for the review.

[kinke]

It'd be nice to include the translator as git submodule of our LLVM fork once we upgrade to v8. It should be as simple as adding the submodule in the right location (project subdir); if the llvm-spirv build target isn't included by default in all, the ninja command line in the CI scripts would need to be extended too.
Then the prebuilt LDC packages will finally feature experimental OpenCL support too (I'm not interested in proprietary CUDA at all).

[thewilsonator]

if the llvm-spirv build target isn't included by default in all

I'm only doing some copying (all of ~3MB) for out-of-tree because I can't be bothered to figure out how to tell lit where the binary is. For in-tree, I'm pretty sure it would be.

[thewilsonator]

This good to go now?

[JohanEngelen]

lgtm

[TurkeyMan]

So this means that all releases of LDC will support SPIR-V output?

[thewilsonator]

It means they can. When we transition to releases with LLVM 8 (I think we release with 6 atm, @kinke?) I assume it will be enabled.

[kinke]

We release with 7.0.1.

[p0nce]

Wow thanks for this, I just discovered LDC could generate OpenCL friendly SPIR-V!

[thewilsonator]

Pleasure, when we get around to releasing with LLVM 15/16 (depending on the maturity of the new SPIR-V backend) we will transition to using that.