explicitly enable MoveOnly feature when building _Concurrency lib

We ran into an issue on minimal stdlib build configurations, because they
pull slightly older toolchains to build and test the stdlib. The noncopyable
`Job` type was adopted immediately after such types were enabled by default,
before the toolchain builds could catch up. Adding this flag to the build of
the `_Concurrency` lib should make things more robust when somewhat older
compilers are building this part of the stdlib.

part of resolving rdar://106849189

Author: kavon

stdlib/public/Concurrency/CMakeLists.txt

@@ -149,6 +149,7 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -parse-stdlib
     -Xfrontend -enable-experimental-concurrency
+    -enable-experimental-feature MoveOnly
     -diagnostic-style swift
     ${SWIFT_RUNTIME_CONCURRENCY_SWIFT_FLAGS}
   ${swift_concurrency_options}