Add invalid swizzle assignment validation

[sudonatalie]

These tests pass with the WIP implementation of swizzle assignment in dawn. This is the last set of tests for the swizzle assignment language feature.

Issue: #4579

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at 3ebf801):

+webgpu:shader,validation,statement,swizzle_assignment:invalid_mixed_letter_schemes:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,statement,swizzle_assignment:invalid_address_of_swizzle_view:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,statement,swizzle_assignment:invalid_index_into_swizzle_view:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 280578 cases, 2321551 subcases
+TOTAL: 280581 cases, 2321554 subcases