Clarify SPIRV Translator dependency and remove custom SPIRV_TRANSLATOR_DIR

[dvrogozh]

We touched these 2 topics in #45 (comment).

1. SPIRV Translator is embedded into LLVM or not?

The default path of common_clang build assumes that https://github.com/KhronosGroup/SPIRV-LLVM-Translator/ will be built and packaged within LLVM build which could result either in separate static library named libLLVMSPIRVLib.a or in a unified LLVM shared library which will include translator library inside.

However, it seems that distributions may incline to build SPIRV Translator as a separate package rather than within LLVM. For example: https://repology.org/project/spirv-llvm-translator/versions - that's what Debian and Ubuntu does.

I would like to use this Issue to collect community feedback on this. @tjaalton, please, comment.

2. How to detect SPIRV Translator on the system?

As of now common_clang already has a path to assure that it works with SPIRV translator as a separate package on the system. If I understand correctly, for that we need to specify 2 options:
-DLLVMSPIRV_INCLUDED_IN_LLVM=OFF -DSPIRV_TRANSLATOR_DIR=/path/to/installed/spirv/translator

While 1st option seems reasonable (though we may wish to consider changing the default after feedback on the 1st question), then 2nd option seems excessive. SPIRV translator provides lib/pkgconfig/LLVMSPIRVLib.pc, thus we can use standard mechanism to detect it on the system rather than introduce custom SPIRV_TRANSLATOR_DIR. Can this change be introduced?

[AlexeySachkov]

SPIRV Translator is embedded into LLVM or not?

It depends on how it was built. I agree that Linux distributives most likely will package it separately. I think that by default we need to assume that SPIRV-LLVM-Translator is not bundled together with LLVM

How to detect SPIRV Translator on the system?

I think we need to state that SPIRV-LLVM-Translator is the same pre-requisite for opencl-clang building as LLVM & Clang and it should be installed on the system. In this case SPIRV_TRANSLATOR_DIR is not needed at all.

However, I would like to leave this option in place, because you may want to build opencl-clang with custom SPIRV-LLVM-Translator or in an environment where it is not installed to the default location on a system (for example there is a similar option for llvm: LLVM_DIR). This option should be optional instead of mandatory.

[dvrogozh]

Yep, that's what I would do. We are on the same page here. I would advice though to avoid overcomplicating build system. It might be reasonable to have SPIRV_TRANSLATOR_DIR, maybe on Windows that would be the right way to do (as an example), but I would double check.