[SPIR-V][vk::SampledTexture] #2. Add .CalculateLevelOfDetailUnclamped() and .Gather() methods for vk::SampledTexture2D type.

[luciechoi]

Part of #7979

Implement methods for vk::SampledTexture2D:

float CalculateLevelOfDetailUnclamped(float2 location);
<Template Type>4 Gather(float2 location, [int2 offset], [uint status]);

[luciechoi]

@s-perron
(I've separated into different PRs as discussed, please see the last two commits)

[s-perron]

This looks good. Nothing too different than what I said on the first PR.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[s-perron]

LGTM but presubmits are still failing.

[s-perron]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).