[stdlib] Float16: Add explicit Sendable conformance to work around swiftinterface issue

Author: lorentey

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -1370,6 +1370,17 @@ public struct ${Self} {
   }
 }
 
+%  if bits == 16:
+// This is a workaround for a compiler bug that omits the macOS 11 availability
+// from the implicit conformance emitted into the generated .swiftinterface
+// file. See https://github.com/apple/swift/pull/36669 for details.
+// FIXME: rdar://76092800
+@available(macOS 11, iOS 14, tvOS 14, watchOS 7, *)
+@available(macOS, unavailable)
+@available(macCatalyst, unavailable)
+extension ${Self}: Sendable { }
+%  end
+
 #endif
 % end
 % end # for bits in all_floating_point_types