Expression evaluation for functions with unknown signatures on works by

Expression evaluation for functions with unknown signatures on works by
inferring the function signature.  This works well where the ABI doesn't
distinguish between variadic and fixed argument lists, but on arm64 the
calling conventions differ.  The default assumption works for fixed argument
lists, but variadic functions require explicit prototypes to be called.

By far the most common case where this is an issue is when attempting to use
 printf().  This change augments the default expression prefix to include a
working variadic prototype for the function.

<rdar://problem/19024779>

llvm-svn: 226744