[clang] [AMDGPU] Pack uniform float structs into vector types (PR #195417)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> Well, we will probably need to put this on hold, since this, including the original PR, is an ABI breaking change. Not sure what would be the best way to move forward with it.

This is fine, we do not have a stable ABI and I don't want to start arbitrarily pretending we do here 

https://github.com/llvm/llvm-project/pull/195417