unittests: adjust for VS2017 15.8+

Repair the VS2017 build similar to the standard library.

Author: compnerd

unittests/runtime/CMakeLists.txt

@@ -85,6 +85,11 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
                              PRIVATE
                                swiftCore_EXPORTS)
 
+  if(SWIFT_HOST_VARIANT STREQUAL WINDOWS)
+    target_compile_definitions(SwiftRuntimeTests PRIVATE
+      _ENABLE_EXTENDED_ALIGNED_STORAGE)
+  endif()
+
   # FIXME: cross-compile for all variants.
   target_link_libraries(SwiftRuntimeTests
     PRIVATE