[Swift in WebKit] Promote VariableNeverMutated and UnusedImportAccess to errors - #77427

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[Swift in WebKit] Promote VariableNeverMutated and UnusedImportAccess to errors
https://bugs.webkit.org/show_bug.cgi?id=327982
rdar://190336417

Reviewed by NOBODY (OOPS!).

Treat the VariableNeverMutated and UnusedImportAccess Swift diagnostic groups
as errors instead of warnings.

In the Xcode build, both are only errors with SDKs that ship Swift 6.4 or later,
since the build still has to support Swift 6.3. With Swift 6.3, the existing
violations can't be fixed (rdar://170907318) or suppressed (`@diagnose` is
unavailable).

In the CMake build, this also fixes NoUsage and NoUseUnstructuredThrowingTask
never being enforced. They were gated on CMAKE_Swift_LANGUAGE_VERSION, which is
never set, and WebKitSwiftFlags.cmake is included before enable_language(Swift),
so no Swift version variable is available at that point anyway. The gate kept
unknown warning groups on older toolchains from failing the build, but that
can no longer happen since Swift warnings are no longer treated as errors, so
remove it to match Xcode.

* Configurations/CommonBase.xcconfig:
* Source/WebGPU/WebGPU/CommandEncoder.swift:
(WebGPU.beginRenderPass(_:)):
* Source/WebKit/UIProcess/API/Swift/WebPage+ImmersiveEnvironment.swift:
* Source/WebKit/UIProcess/Cocoa/TextExtraction/WKWebView+TextExtraction.swift:
* Source/WebKit/_WebKit_SwiftUI/CrossImportOverlay.swift:
* Source/cmake/WebKitSwiftFlags.cmake:

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