build: use the same pattern for detecting Darwin platforms

The host platform should be using `CMAKE_SYSTEM_NAME STREQUAL Darwin`.
However, we currently drive the host side of the compilation against
custom variables.  This makes the migration simpler by ensuring that the
entire file uses the same pattern.

Since `is_darwin_based_sdk` is now used only in the standard library
build, sink it to the standard library build.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -75,25 +75,10 @@ function(_set_target_prefix_and_suffix target kind sdk)
   endif()
 endfunction()
 
-function(is_darwin_based_sdk sdk_name out_var)
-  if ("${sdk_name}" STREQUAL "OSX" OR
-      "${sdk_name}" STREQUAL "IOS" OR
-      "${sdk_name}" STREQUAL "IOS_SIMULATOR" OR
-      "${sdk_name}" STREQUAL "TVOS" OR
-      "${sdk_name}" STREQUAL "TVOS_SIMULATOR" OR
-      "${sdk_name}" STREQUAL "WATCHOS" OR
-      "${sdk_name}" STREQUAL "WATCHOS_SIMULATOR")
-    set(${out_var} TRUE PARENT_SCOPE)
-  else()
-    set(${out_var} FALSE PARENT_SCOPE)
-  endif()
-endfunction()
-
 # Usage:
 # _add_host_variant_c_compile_link_flags(name)
 function(_add_host_variant_c_compile_link_flags name)
-  is_darwin_based_sdk("${SWIFT_HOST_VARIANT_SDK}" IS_DARWIN)
-  if(IS_DARWIN)
+  if(SWIFT_HOST_VARIANT_SDK IN_LIST SWIFT_APPLE_PLATFORMS)
     set(DEPLOYMENT_VERSION "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_DEPLOYMENT_VERSION}")
   endif()
 
@@ -107,7 +92,7 @@ function(_add_host_variant_c_compile_link_flags name)
 
   set(_sysroot
     "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}")
-  if(IS_DARWIN)
+  if(SWIFT_HOST_VARIANT_SDK IN_LIST SWIFT_APPLE_PLATFORMS)
     target_compile_options(${name} PRIVATE -isysroot;${_sysroot})
   elseif(NOT SWIFT_COMPILER_IS_MSVC_LIKE AND NOT "${_sysroot}" STREQUAL "/")
     target_compile_options(${name} PRIVATE --sysroot=${_sysroot})
@@ -122,7 +107,7 @@ function(_add_host_variant_c_compile_link_flags name)
     endif()
   endif()
 
-  if(IS_DARWIN)
+  if(SWIFT_HOST_VARIANT_SDK IN_LIST SWIFT_APPLE_PLATFORMS)
     # We collate -F with the framework path to avoid unwanted deduplication
     # of options by target_compile_options -- this way no undesired
     # side effects are introduced should a new search path be added.

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -2,6 +2,20 @@
 include(AddSwift)
 include(SwiftSource)
 
+function(is_darwin_based_sdk sdk_name out_var)
+  if ("${sdk_name}" STREQUAL "OSX" OR
+      "${sdk_name}" STREQUAL "IOS" OR
+      "${sdk_name}" STREQUAL "IOS_SIMULATOR" OR
+      "${sdk_name}" STREQUAL "TVOS" OR
+      "${sdk_name}" STREQUAL "TVOS_SIMULATOR" OR
+      "${sdk_name}" STREQUAL "WATCHOS" OR
+      "${sdk_name}" STREQUAL "WATCHOS_SIMULATOR")
+    set(${out_var} TRUE PARENT_SCOPE)
+  else()
+    set(${out_var} FALSE PARENT_SCOPE)
+  endif()
+endfunction()
+
 function(add_dependencies_multiple_targets)
   cmake_parse_arguments(
       ADMT # prefix