Various fixes to symbols used for Obj-C x86_64 metadata.

 - Changed method names to match gcc (categories names still aren't
   mangled in).

 - Expose correct name for class and metadata symbols (although
   -fvisibility=hidden isn't yet correct).

 - Remove several things from llvm.used that didn't need to be there
   (I suspect this can still be trimmed).

 - Don't use asm-prefix extension for _objc_empty_{cache,vtable} (not
   needed).

 - Hide EH type class info with -fvisibility=hidden

 - Change setGlobal[Option]Visibility to not change the visibility of
   functions with internal linkage.

llvm-svn: 68510