This patch makes Clang predefine `_CRT_USE_BUILTIN_OFFSETOF` in
MS-compatible modes. The macro can make the `offsetof` provided by MS
UCRT's `<stddef.h>` to select the `__builtin_offsetof` version, so with
it Clang (Clang-cl) can directly consume UCRT's `offsetof`.
MSVC predefines the macro as `1` since at least VS 2017 19.14, but I
think it's also OK to define it in "older" compatible modes.
Fixes#59689.
