swift

830d76be - Embedded: Fix Observation configure error in embedded-only stdlib (#91318)

Embedded: Fix Observation configure error in embedded-only stdlib (#91318) Embedded-only stdlib builds configure with an empty `SWIFT_SDKS`, so the non-embedded `swiftObservation` target never runs and never registers `ThreadLocal.cpp` with LLVM's source-list check. Marking it optional matches what Concurrency already does for `ExecutorImpl.cpp`. rdar://184326879