Only register XPC interest when a delegate exists #102155

Job	Run time
Build framework (macos-release, -sdk macosx -configuration Release)	15m 45s
Build framework (macos-release, -sdk macosx -configuration Release)	15m 45s
Build framework (ios-release, -sdk iphoneos -configuration Release)	12m 32s
Build framework (ios-release, -sdk iphoneos -configuration Release)	12m 32s
Build framework (tvos-debug, -sdk appletvos -configuration Debug)	10m 7s
Build framework (tvos-debug, -sdk appletvos -configuration Debug)	10m 7s
Build framework (watchos-debug, -sdk watchos -configuration Debug)	16m 9s
Build framework (watchos-debug, -sdk watchos -configuration Debug)	16m 9s
Run framework tests (asan, -enableAddressSanitizer YES -enableUndefinedBehaviorSanitizer YES)	35m 28s
Run framework tests (asan, -enableAddressSanitizer YES -enableUndefinedBehaviorSanitizer YES)	35m 28s
Run framework tests (tsan, -enableThreadSanitizer YES)	38m 47s
Run framework tests (tsan, -enableThreadSanitizer YES)	31m 56s
Run framework tests (leaks, ENABLE_LEAK_DETECTION=1)	38m 21s
Run framework tests (leaks, ENABLE_LEAK_DETECTION=1)	30m 11s
	5h 19m 17s

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