[clang] [clang] Allow C-style casts in constexpr in MS compatible mode (PR #197005)

[James Y Knight via cfe-commits]

jyknight wrote:

Looks like winnt.h already says, since a long time ago:
```
#ifdef __has_builtin
#if __has_builtin(__builtin_offsetof)
#define FIELD_OFFSET(type, field)    ((LONG)__builtin_offsetof(type, field))
[...]
```

Is the problem that someone's copied this define into another file? If so, can that other file just be updated correspondingly, with the same change?

https://github.com/llvm/llvm-project/pull/197005