[PATCH] Always emit DW_AT_declaration attribute when the variable isn't a definition.

[Frederic Riss]

It's not that we want declarations for all globals, but sometimes a declaration is all we have:

```
namespace ns {
extern int A;
}
using ns::A;
int foo(int Arg = A) { return Arg; }

```

In the above, if we want to emit the imported_declaration or the default argument, we want to have a declaration only DIE for the global.

http://reviews.llvm.org/D5382