platform.h: Always compile in MSVC mode if _MSC_VER is set

[romanrexc]

This fixes embedding Julia on Windows when compiling with clang-cl.

[ViralBShah]

@barche Thoughts here?

[barche]

I have never tried clang-cl, but I can imagine the old logic would break it, and this fix only changes the logic for the specific case of clang-cl, so this should be fine!

[romanrexc]

It breaks it here:

In file included from julia-win64\include\julia\julia.h:20:
In file included from julia-win64\include\julia\libsupport.h:12:
julia-win64\include\julia\ios.h(88,30): error: unknown type name 'ios_t'
   88 | JL_DLLEXPORT size_t ios_read(ios_t *s, char *dest, size_t n) JL_NOTSAFEPOINT;