Move availability args to response file

The availability flags keep increasing the number of characters on the
command line overflowing the maximum character length on Windows and
making the commands harder to read. This patch moves the generated
arguments into a separate response file so that they can continue to
grow without running into argument length limitations.

Author: etcwilde

Runtimes/Core/cmake/modules/AvailabilityMacros.cmake

@@ -4,7 +4,7 @@ configure_file("${SwiftCore_SWIFTC_SOURCE_DIR}/utils/availability-macros.def"
 file(STRINGS "${CMAKE_CURRENT_BINARY_DIR}/availability-macros.def" availability_defs)
 list(FILTER availability_defs EXCLUDE REGEX "^\\s*(#.*)?$")
 foreach(def ${availability_defs})
-  add_compile_options("$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -define-availability -Xfrontend \"${def}\">")
+  list(APPEND availability_definitions "-Xfrontend -define-availability -Xfrontend \"${def}\"")
 
   if("${def}" MATCHES "SwiftStdlib .*")
     # For each SwiftStdlib x.y, also define StdlibDeploymentTarget x.y, which,
@@ -31,6 +31,12 @@ foreach(def ${availability_defs})
         endif()
       endif()
     endif()
-    add_compile_options("$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xfrontend -define-availability -Xfrontend \"${current}\">")
+    list(APPEND availability_definitions "-Xfrontend -define-availability -Xfrontend \"${current}\"")
   endif()
 endforeach()
+
+list(JOIN availability_definitions "\n" availability_definitions)
+file(GENERATE
+  OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/availability-macros.rsp"
+  CONTENT "${availability_definitions}")
+add_compile_options("$<$<COMPILE_LANGUAGE:Swift>:SHELL:${CMAKE_Swift_RESPONSE_FILE_FLAG}${CMAKE_CURRENT_BINARY_DIR}/availability-macros.rsp>")