Add a step to test compiling generated code in Swift 6 mode. #1587

Job	Run time
swift-format Check	36s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	9m 43s
Api Breakage Compared to main branch	1m 29s
core (5.10.1-noble, ref/heads/main)	10m 18s
fuzzing_regressions (debug)	3m 36s
sanitizer_testing (address, debug)	2m 40s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	8m 33s
fuzzing_regressions (release)	3m 9s
sanitizer_testing (address, release)	5m 17s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	7m 30s
sanitizer_testing (thread, debug)	2m 43s
sanitizer_testing (thread, release)	4m 57s
	1h 0m 31s

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