Meta-issue for cast insertion in unwritable files

[mattmccutchen-cci]

This meta-issue tracks all the known bugs that can cause attempted cast insertion in unwritable files and, ultimately, changing that case from a warning back to an error once the bugs are fixed.

• 3C inserts cast into unwritable inline function body that calls function pointer with itype #423 (first to be identified and has additional discussion of the general problem)
• 3C inserts unwritable casts into stdio.h and corecrt_wstdio.h on Windows #449