IRgen: Support user defined attributes on block runtime functions.

 - This issue here is that /usr/include/Blocks.h wants to define some of the
   block runtime globals as weak, depending on the target. This doesn't work in
   Clang because we aren't using the AST decl for these globals.

 - The fix is a pretty gross hack which just watches all the decls for the
   specific blocks globals we need to know about; if we see one we use it,
   otherwise we use the hand coded type.

   In time, I would like to clean this up by changing IRgen to ask Sema/AST for
   the decl, which would then be lazily loaded from the builtin table if
   necessary. This could be used in a whole host of places in IRgen and would
   get rid of a lot of grotty hand coding of LLVM IR; however, we need some
   extra Sema support for this as well as support for builtin global variables.

llvm-svn: 108482