Make ProcessDeclAttributes walk the declarator structure pulling

decl attributes out of the various places they can hide.  This makes
us correctly reject things like this:

t.c:2:22: error: mode attribute only supported for integer and floating-point types
  int **__attribute((mode(HI)))* i32;
                     ^

because you can't make a pointer be HImode.

llvm-svn: 52876