[embedded] Make stdlib APIs automatically available on older Darwin versions in Embedded Swift

Author: kubamracek

stdlib/public/core/CMakeLists.txt

@@ -434,6 +434,15 @@ if(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB)
   set(SWIFT_STDLIB_ENABLE_OBJC_INTEROP OFF)
   set(SWIFT_STDLIB_ENABLE_VECTOR_TYPES OFF)
 
+  # Under Embedded Swift, all stdlib APIs should be available always. Replace
+  # all availability macros with 10.13.
+  set(SWIFT_STDLIB_AVAILABILITY_DEFINITIONS_EMBEDDED)
+  foreach(def ${SWIFT_STDLIB_AVAILABILITY_DEFINITIONS})
+    string(REGEX REPLACE ":.*" ":macOS 10.13" replaced "${def}")
+    list(APPEND SWIFT_STDLIB_AVAILABILITY_DEFINITIONS_EMBEDDED "${replaced}")
+  endforeach()
+  set(SWIFT_STDLIB_AVAILABILITY_DEFINITIONS "${SWIFT_STDLIB_AVAILABILITY_DEFINITIONS_EMBEDDED}")
+
   foreach(entry ${EMBEDDED_STDLIB_TARGET_TRIPLES})
     string(REGEX REPLACE "[ \t]+" ";" list "${entry}")
     list(GET list 0 arch)