wgsl: Fix field semantics even if input struct doesn't need flattening

[aleino-nv]

This fixes an issue where location indices of stage inputs were repeated

#5633

[aleino-nv]

Falcor tests failed:

  internal/renderpasses/test_InternalPathTracer_vulkan         : FAILED (600.0 s)
    Process killed due to timeout
    View test at: http://SlangWin5:8080//unknown/internal/renderpasses/test_InternalPathTracer_vulkan

[jkwak-work]

I feel uncomfortable with this change.

The code is copied exactly from slang-ir-metal-legalize.cpp
And with this PR, WGPU side works but Metal side is failing.

I suggest to investigate why Metal is failing and fix it properly before merging this change.

[csyonghe]

I feel like metal needs the same treatment.

[jkwak-work]

I feel like metal needs the same treatment.

Yes, that must be it.

[aleino-nv]

I feel like metal needs the same treatment.

Yes, that must be it.

[aleino-nv]

I feel like metal needs the same treatment.

Here is a PR
#5666

[aleino-nv]

I feel like metal needs the same treatment.

Here is a PR #5666

Ok the PR fails, I filed #5667.
The render0 test is also broken for Metal, so it's probably best to fix that one first, and then enable this test.