Make SWIFT_STDLIB_ENABLE_LTO flag apply LTO to C/C++ compile flags too (#38959)

Author: kubamracek

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1150,9 +1150,7 @@ function(_add_swift_target_library_single target name)
     set(analyze_code_coverage "${SWIFT_ANALYZE_CODE_COVERAGE}")
   endif()
 
-  if (NOT SWIFTLIB_SINGLE_TARGET_LIBRARY)
-    set(lto_type "${SWIFT_STDLIB_ENABLE_LTO}")
-  endif()
+  set(lto_type "${SWIFT_STDLIB_ENABLE_LTO}")
 
   _add_target_variant_c_compile_flags(
     SDK "${SWIFTLIB_SINGLE_SDK}"