[WebGPU] Hook up nullability attributes

[litherum]

6ed99c2

[WebGPU] Hook up nullability attributes
https://bugs.webkit.org/show_bug.cgi?id=257675
rdar://110202947

Reviewed by NOBODY (OOPS!).

WebGPU.h already has macros to specify nullability. This patch hooks them up to the
appropriate clang attributes.

WebGPU.h's macros don't work out of the box; I opened
webgpu-native/webgpu-headers#190 about fixing the problems
with it.

* Source/WebCore/PAL/Configurations/PAL.xcconfig:
* Source/WebGPU/Configurations/Base.xcconfig:
* Source/WebGPU/Configurations/WebGPU.xcconfig:
* Source/WebGPU/WebGPU/Instance.mm:
(wgpuInstanceRequestAdapter):
* Source/WebGPU/WebGPU/WebGPU.h:

6ed99c2

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[webkit-early-warning-system]

EWS run on current version of this PR (hash 6ed99c2)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2
	✅ 🛠 tv-sim
	✅ 🛠 watch
	✅ 🛠 watch-sim