Move the fixit for -Wformat-security to a note.

Move the fixit for -Wformat-security to a note.

r263299 added a fixit for the -Wformat-security warning, but that runs
into complications with our guideline that error recovery should be done
as-if the fixit had been applied. Putting the fixit on a note avoids that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263584 91177308-0d34-0410-b5e6-96231b3b80d8