Add a step to test compiling generated code in Swift 6 mode. #1589

Job	Run time
Api Breakage Compared to main branch	1m 30s
swift-format Check	37s
fuzzing_regressions (debug)	3m 34s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	14m 50s
fuzzing_regressions (release)	3m 6s
core (5.10.1-noble, ref/heads/main)	15m 45s
sanitizer_testing (address, debug)	2m 34s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	14m 28s
sanitizer_testing (address, release)	5m 12s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	13m 42s
sanitizer_testing (thread, debug)	2m 46s
sanitizer_testing (thread, release)	4m 50s
	1h 22m 54s

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