3C inserts unwritable casts into `stdio.h` and `corecrt_wstdio.h` on Windows

[mattmccutchen-cci]

On Windows, the 3C regression tests that #include <stdio.h> generate unwritable change errors by trying to insert casts into stdio.h and corecrt_wstdio.h.

I've put the old and new versions of the files in /home/engineers/windows-unwritable-casts-20210225 on gamera (you may need to re-login to have access to this directory); I'm not posting them here due to conceivable concerns about Microsoft's copyright to the files. I haven't done any further investigation yet.

Is it time to just make cast insertion into an unwritable file a warning for now?

[mwhicks1]

Yeah, let's make it a warning.