Add a step to test compiling generated code in Swift 6 mode. #1590

Job	Run time
swift-format Check	38s
fuzzing_regressions (debug)	3m 39s
Api Breakage Compared to main branch	1m 40s
fuzzing_regressions (release)	2m 59s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	15m 27s
sanitizer_testing (address, debug)	2m 37s
core (5.10.1-noble, ref/heads/main)	16m 24s
sanitizer_testing (address, release)	5m 10s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	14m 39s
sanitizer_testing (thread, debug)	2m 56s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	13m 34s
sanitizer_testing (thread, release)	4m 46s
	1h 24m 29s

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