clang/win: Add __readfsdword to intrin.h

When using __readfsdword(), clang used to warn that one has
to include <intrin.h> -- no matter if that was already included
or not.

Now it only warns if it's not yet included.

To verify that this was the only intrin with this problem, I ran:

    $ for f in $(grep intrin.h clang/include/clang/Basic/BuiltinsX86* |
                 egrep -o '\([^,]+,' | egrep -o '[^(,]*'); do
        if ! grep -q $f clang/lib/Headers/intrin.h; then echo $f; fi;
      done

This printed 9 more functions, but those are all in emmintrin.h,
xsaveintrin.h (which are included by intrin.h based on /arch: flags).
So this is indeed the only built-in that was missing in intrin.h.

Fixes PR51188.

Differential Revision: https://reviews.llvm.org/D109085