Turn on strncat-size warning implemented a while ago.

Warns on anti-patterns/typos in the 'size' argument to strncat. The
correct size argument should look like the following:
 - strncat(dst, src, sizeof(dst) - strlen(dest) - 1);

We warn on: 
 - sizeof(dst)
 - sizeof(src)
 - sizeof(dst) - strlen(dst)
 - sizeof(src) - anything

(This has been implemented in void Sema::CheckStrncatArguments().)

llvm-svn: 161440