build: adjust the flags addition for the standard library

The standard library cannot be built with MSVC's compiler.  However, if
it is built with the toolchain which uses MSVC, it would fail to add
this flag.  With recent VS2022 toolsets, this is a problem as we would
fail to be able to use the `offsetof` macro.  Unconditionally apply the
definition when building the standard library.

Author: compnerd

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -215,9 +215,7 @@ function(_add_target_variant_c_compile_flags)
     list(APPEND result "-DLLVM_ON_WIN32")
     list(APPEND result "-D_CRT_SECURE_NO_WARNINGS")
     list(APPEND result "-D_CRT_NONSTDC_NO_WARNINGS")
-    if(NOT "${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")
-      list(APPEND result "-D_CRT_USE_BUILTIN_OFFSETOF")
-    endif()
+    list(APPEND result "-D_CRT_USE_BUILTIN_OFFSETOF")
     # TODO(compnerd) permit building for different families
     list(APPEND result "-D_CRT_USE_WINAPI_FAMILY_DESKTOP_APP")
     if("${CFLAGS_ARCH}" MATCHES arm)