BridgeJS: Deduplicate closure signature collection logic #2202

Job	Run time
check-bridgejs-generated	2m 41s
Test BridgeJS against Swift versions (swift:6.2, 602.0.0)	3m 1s
build-examples	25m 12s
Build for native target (macos-15, Xcode_16.4)	2m 26s
format	32s
Test BridgeJS against Swift versions (swift:6.1.2, 601.0.0)	2m 57s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	13m 43s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	17m 17s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 8s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	12m 25s
deploy-examples	0s
	1h 33m 22s

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