BridgeJS: Fix protocol existential lowering in Swift-to-JS direction #2467

Job	Run time
check-bridgejs-generated	2m 35s
format	31s
prettier	9s
build-examples	7m 3s
Test BridgeJS against Swift versions (swift:6.1.2, 601.0.0)	3m 0s
Test BridgeJS against Swift versions (swift:6.2, 602.0.0)	3m 12s
Build for native target (macos-15, Xcode_16.4)	2m 36s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	7m 3s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	7m 2s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	7m 3s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	7m 4s
deploy-examples	1s
	47m 19s

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