Without this patch clang warns on

struct __attribute__((visibility("hidden"))) zed {
};
struct __attribute__((visibility("hidden"))) zed;

Which is a bit silly and got a lot noisier now that we correctly handle
visibility pragmas. This patch fixes that and also has some extra quality
improvements:

* We now produce an error instead of a warning for

struct __attribute__((visibility("hidden"))) zed {
};
struct __attribute__((visibility("default"))) zed;

* The "after definition" warning now points to the new attribute that is
  ignored instead of pointing to the declaration.

llvm-svn: 160227