[emdawnwebgpu] Fix StringView argument signatures for MEMORY64

[slowriot]

The signature generator treats by-value WGPUStringView arguments as unsupported structures, causing their generated __sig entries to be commented out.

Under MEMORY64, this prevents Emscripten from generating BigInt-to-Number pointer conversions for affected JavaScript bindings.

Calling PushDebugGroup or InsertDebugMarker consequently throws:

TypeError: Cannot mix BigInt and other types, use explicit conversions

The exception occurs in makeStringFromStringView when it performs Number arithmetic on the incoming BigInt address.

Recognise by-value WGPUStringView arguments as pointers, matching their indirect passing convention in the Wasm C ABI. This generates the required vpp signatures for the debug-group and marker functions. Structure return values and other structure types remain unchanged.

Validation:

• Regenerated emdawnwebgpu signatures and verified all eight PushDebugGroup / InsertDebugMarker bindings emit vpp.
• Checked that StringView return values and other structure types retain their existing handling.
• Confirmed that applying the equivalent generated-signature changes fixes the crash in a wasm64 WebGPU application.

[github-actions]

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[kainino0x]

Review comments on https://dawn-review.googlesource.com/c/dawn/+/342595