[spirv] Fix storage class for ptr-to-ptr in Vulkan 1.2.

[ehsannas]

Also fixes #3340

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.4055 completed (commit d1afd116d4 by @ehsannas)

[manon-traverse]

I was about to submit an issue about a crash during the compilation of a shader, but we found out that this PR solves our issue.
I created the following repro shader that breaks on master, but works on this branch

ByteAddressBuffer g_byteAddressBuffer[] : register(t0, space3);
[numthreads(1,1,1)]
void main() {
    ByteAddressBuffer flat_bucket_indices = g_byteAddressBuffer[0];
}

dxc -spirv -Tcs_6_5 -fspv-target-env=vulkan1.2 -Emain -fvk-use-dx-layout repro.hlsl

Specifically assigning to the flat_bucket_indices is what causes the crash during compilation. Sampling from the buffer directly without assigning it as a variable worked as a workaround for us.

EDIT: Made the repro shader smaller and added compile command

[Jasper-Bekkers]

Further reduced version of the above code sample:

ByteAddressBuffer g_byteAddressBuffer[] : register(t0, space3);
[numthreads(1,1,1)]
void main() {
    ByteAddressBuffer flat_bucket_indices = g_byteAddressBuffer[0];
}

dxc -spirv -Tcs_6_5 -fspv-target-env=vulkan1.2 -Emain -fvk-use-dx-layout code.hlsl

[ehsannas]

Thanks @max-traverse @Jasper-Bekkers I've added this as an additional test case.

@jaebaek PTAL.

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✅ Build DirectXShaderCompiler 1.0.4119 completed (commit 4b6ee64156 by @ehsannas)

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✅ Build DirectXShaderCompiler 1.0.4157 completed (commit ebaa3ee014 by @ehsannas)