[llvm] [RFC][ThinLTO][AMDGPU] Add `AMDGPU_SUMMARY` bitcode block for ThinLTO (PR #194545)

[Teresa Johnson via llvm-commits]

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+; Verify that the AMDGPU_SUMMARY block round-trips through bitcode.
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teresajohnson wrote:

This isn't quite round-tripping, as the reader isn't invoked.

https://github.com/llvm/llvm-project/pull/194545