[clang] [llvm] [HLSL] Add semantic signature packing flags and invoke packing (PR #226554)

[via llvm-commits]

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+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.3-library -emit-llvm -finclude-default-header -disable-llvm-passes -fdx-semantic-signature-packing-mode=prefix-stable -DSTACKED -o /dev/null -verify %s
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joaosaffran wrote:

nit: I found that a bit hard to follow and map what happens when. Maybe move those into 3 distinct tests?

https://github.com/llvm/llvm-project/pull/226554