Make sure we define availability macros for standalone library builds

Fixes rdar://85639438.

Author: DougGregor

cmake/modules/StandaloneOverlay.cmake

@@ -73,6 +73,9 @@ option(SWIFT_ENABLE_MODULE_INTERFACES
 set(SWIFT_STDLIB_BUILD_TYPE "${CMAKE_BUILD_TYPE}" CACHE STRING
   "Build type for the Swift standard library and SDK overlays.")
 
+file(STRINGS "../../utils/availability-macros.def" SWIFT_STDLIB_AVAILABILITY_DEFINITIONS)
+list(FILTER SWIFT_STDLIB_AVAILABILITY_DEFINITIONS EXCLUDE REGEX "^\\s*(#.*)?$")
+
 set(SWIFT_DARWIN_SUPPORTED_ARCHS "" CACHE STRING
   "Semicolon-separated list of architectures to configure on Darwin platforms. \
 If left empty all default architectures are configured.")