PR4290: Handle vfprintf in a way that doesn't give any diagnostics for

valid declarations and doesn't give an error for autoconf-style invalid 
redeclarations.  This isn't quite ideal, but I don't see any other way 
easy way to handle it.  (The only thing I can think of that's reasonably 
general is adding a new builtin type FILETy which is only compatible 
with a type equivalent to FILE, and that seems like overkill.)

llvm-svn: 72760