linker/test: prepare for SPIR-T compatibility.

[eddyb]

This is a prerequisite for:

• Minimally integrate SPIR-T (opt-in via RUSTGPU_CODEGEN_ARGS=--spirt). #940 (comment)
  ^{(the link goes to my comment explanation the plan that this PR is a part of)}

Each commit is an individual step and can be reviewed as such, but the notable ones are:

• removed "decoration groups" (we don't generate them, they're deprecated, SPIR-T does not support them)
• added a mechanism to keep Exports around long enough so SPIR-T can understand what these tests want
• flipped on DCE (which will remove otherwise-unused definitions) and added more Exports to keep things alive

Overall I really have not liked these tests and they're the main reason the SPIR-T PR got delayed so much.
I opened an issue about moving away from these unit tests, but sadly I don't know how much effort it'd be:

• We should replace rustc_codegen_spirv::linker::test unit tests with compiletest ones. #957