build: correct handling of argument generation

This adjusts the target specific argument handling to use the same logic
as the toolchain, and in doing so, silences the spurious warnings when
building with a MSVC toolchain.

Author: compnerd

cmake/modules/AddSwiftUnittests.cmake

@@ -57,9 +57,11 @@ function(add_swift_unittest test_dirname)
       _ENABLE_EXTENDED_ALIGNED_STORAGE)
   endif()
 
-  if(SWIFT_USE_LINKER)
-    set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
-      LINK_FLAGS " -fuse-ld=${SWIFT_USE_LINKER}")
+  if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
+    if(SWIFT_USE_LINKER)
+      target_link_options(${test_dirname} PRIVATE
+        -fuse-ld=${SWIFT_USE_LINKER}$<$<STREQUAL:${CMAKE_HOST_SYSTEM_NAME},Windows>:.exe>)
+    endif()
   endif()
 
   if(SWIFT_ANALYZE_CODE_COVERAGE)