[Sema] Consider all format_arg attributes.

If a function has multiple format_arg attributes, clang only considers
the first it finds (because AttributeLists are in reverse order, not
necessarily the textually first) and ignores all others.

Loop over all FormatArgAttr to print warnings for all declared
format_arg attributes.

For instance, libintl's ngettext (select plural or singular version of
format string) has two __format_arg__ attributes.

Differential Revision: https://reviews.llvm.org/D48734

llvm-svn: 336239