[Build] Fix the minimal-stdlib build by setting a deployment version.

We need to set a deployment version, not just blank, because otherwise
we don't know the correct deployment target to use for the standard
library.

Author: al45tair

cmake/modules/DarwinSDKs.cmake

@@ -22,6 +22,8 @@ is_sdk_requested(FREESTANDING swift_build_freestanding)
 if(swift_build_freestanding AND (SWIFT_FREESTANDING_FLAVOR STREQUAL "apple"))
   set(SWIFT_FREESTANDING_SDK "" CACHE STRING
       "Which SDK to use when building the FREESTANDING stdlib")
+  set(SWIFT_FREESTANDING_DEPLOYMENT_VERSION "" CACHE STRING
+      "The deployment version to use when building the FREESTANDING stdlib")
   set(SWIFT_FREESTANDING_TRIPLE_NAME "" CACHE STRING
       "Which triple name (e.g. 'none-macho') to use when building the FREESTANDING stdlib")
   set(SWIFT_FREESTANDING_MODULE_NAME "" CACHE STRING
@@ -31,7 +33,7 @@ if(swift_build_freestanding AND (SWIFT_FREESTANDING_FLAVOR STREQUAL "apple"))
   set(SWIFT_FREESTANDING_ARCHS "" CACHE STRING
       "Which architectures to build when building the FREESTANDING stdlib")
   configure_sdk_darwin(
-      FREESTANDING "FREESTANDING" ""
+      FREESTANDING "FREESTANDING" "${SWIFT_FREESTANDING_DEPLOYMENT_VERSION}"
       "${SWIFT_FREESTANDING_SDK}"
       "${SWIFT_FREESTANDING_TRIPLE_NAME}" "${SWIFT_FREESTANDING_MODULE_NAME}"
       "${SWIFT_FREESTANDING_AVAILABILITY_NAME}" "${SWIFT_FREESTANDING_ARCHS}")

utils/swift_build_support/swift_build_support/products/minimalstdlib.py

@@ -67,6 +67,8 @@ def build(self, host_target):
         self.cmake_options.define(
             'SWIFT_FREESTANDING_MODULE_NAME:STRING', 'macos')
         self.cmake_options.define('SWIFT_FREESTANDING_SDK:STRING', 'macosx')
+        self.cmake_options.define('SWIFT_FREESTANDING_DEPLOYMENT_VERSION:STRING',
+                                  '11.0')
         self.cmake_options.define(
             'SWIFT_FREESTANDING_TRIPLE_NAME:STRING', 'macosx11.0')
         self.cmake_options.define(