[Clang] Fix: Restore warning inadvertently removed by D126061.

Before D126061, Clang would warn about this code

```
struct X {
    [[deprecated]] struct Y {};
};
```

with the warning

    attribute 'deprecated' is ignored, place it after "struct" to apply attribute to type declaration

D126061 inadvertently caused this warning to no longer be emitted. This patch
restores the previous behavior.

The reason for the bug is that after D126061, C++11 attributes applied to a
member declaration are no longer placed in `DS.getAttributes()` but are instead
tracked in a separate list (`DeclAttrs`). In the case of a free-standing
decl-specifier-seq, we would simply ignore the contents of this list. Instead,
we now pass the list on to `Sema::ParsedFreeStandingDeclSpec()` so that it can
issue the appropriate warning.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D128499