[llvm] [SPIRV] Add explicit layout (PR #135789)

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy wrote:

> @VyacheslavLevytskyy I am interested in your thought on the changes to SPIRVIRMapping.Thanks.

Sure, I'm reviewing right now, by a coincidence :)

https://github.com/llvm/llvm-project/pull/135789