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* Initial work on direct emission of SPIR-V
This change adds a first vertical slice of support for emitting SPIR-V code directly from the Slang IR, instead of generating it indirectly via GLSL.
This work isn't usable for anything valuable right now; the goal is just to get something checked in that we can incrementally extend over time.
When invoking `slangc`, the `-emit-spirv-directly` option can be used to turn on the new code path.
I have not bothered to add an equivalent API option, because this flag is only intended to be used for testing in the immediate future.
The existing `emitEntryPoint()` function has become `emitEntryPointSource()` to more accurately reflect its role in a world where we can also emit entry points to a binary format.
Much of the logic that was inside `emitEntryPoint()` had to do with linking and then optimizing/transforming Slang IR code to get it ready for emission on a particular target.
This logic has been factored into a new `linkAndOptimizeIR()` function that can be shared between the path that emits source and the new one that emits SPIR-V.
The meat of the change is then the `emitSPIRVFromIR()` function in `slang-emit-spirv.cpp`, which is called *after* all the optimizations and transformations have been applied to the Slang IR to get it ready.
Rather than repeat myself here, I will try to make the comments in `slang-emit-spirv.cpp` usable as documentation of the approach being taken.
Smaller notes:
* I've included a test case that compares `slangc` output directly to expected SPIR-V. This is perhaps not an ideal plan for how to test SPIR-V emission going forward, but it suffices for now.
* The `external/` directory needed to be added to the include dirs for the `slang` project so that the new code can depend on the SPIR-V header.
* In `slang-ir-link`, the direct SPIR-V generation path means that we now link with a target of SPIR-V instead of GLSL. In principle this can be used to ensure that appropriate variants of intrinsics are selected based on the knowledge that we are emitting SPIR-V. In practice, that isn't being used at all.
* Fixup: path for SPIR-V headers
While working on this PR I used a copy of `spirv.h` that I placed into the repository tree manually, but since I started the work we ended up with SPIR-V headers in our tree anyway, albeit at a different path.
This change tries to fix things up so that my code uses the headers that were already placed in the repository.
* fixup; 64-bit build issue
* fixup: typo fixes based on review
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