[cmake] Update OPTIONAL install products to be Darwin-only

Per review feedback.

Author: benlangmuir

cmake/modules/AddSwift.cmake

@@ -1974,6 +1974,12 @@ function(add_swift_target_library name)
             WORLD_READ)
       endif()
 
+      set(optional_arg)
+      if(sdk IN_LIST SWIFT_APPLE_PLATFORMS)
+        # Allow installation of stdlib without building all variants on Darwin.
+        set(optional_arg "OPTIONAL")
+      endif()
+
       if(sdk STREQUAL WINDOWS AND CMAKE_SYSTEM_NAME STREQUAL Windows)
         swift_install_in_component("${SWIFTLIB_INSTALL_IN_COMPONENT}"
           TARGETS ${name}-windows-${SWIFT_PRIMARY_VARIANT_ARCH}
@@ -1986,7 +1992,7 @@ function(add_swift_target_library name)
             FILES "${UNIVERSAL_LIBRARY_NAME}"
             DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/${resource_dir}/${resource_dir_sdk_subdir}"
             PERMISSIONS ${file_permissions}
-            OPTIONAL)
+            "${optional_arg}")
       endif()
       if(sdk STREQUAL WINDOWS)
         foreach(arch ${SWIFT_SDK_WINDOWS_ARCHITECTURES})
@@ -2034,7 +2040,7 @@ function(add_swift_target_library name)
               OWNER_READ OWNER_WRITE
               GROUP_READ
               WORLD_READ
-            OPTIONAL)
+            "${optional_arg}")
       endif()
 
       # Add Swift standard library targets as dependencies to the top-level

cmake/modules/SwiftSource.cmake

@@ -347,10 +347,16 @@ function(_compile_swift_files
     list(APPEND module_outputs "${interface_file}")
   endif()
 
+  set(optional_arg)
+  if(sdk IN_LIST SWIFT_APPLE_PLATFORMS)
+    # Allow installation of stdlib without building all variants on Darwin.
+    set(optional_arg "OPTIONAL")
+  endif()
+
   swift_install_in_component("${SWIFTFILE_INSTALL_IN_COMPONENT}"
     FILES ${module_outputs}
     DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/swift/${library_subdir}"
-    OPTIONAL)
+    "${optional_arg}")
 
   set(line_directive_tool "${SWIFT_SOURCE_DIR}/utils/line-directive")
   set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc")