[Bug lto/67357] -Wodr warnings from types in anonymous namespace

["rogero at howzatt dot demon.co.uk" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67357

--- Comment #2 from Roger Orr <rogero at howzatt dot demon.co.uk> ---
The fault still occurs on gcc-5-branch, does not occur on gcc trunk.

I have also found that making a small change to the example in comment 5 of PR
66180 makes that fail with gcc 5.2.0: adding const to the type used for the
unique pointers.

The modified example becomes:

$ cat foo1.cpp
#include <memory>
namespace {
class A {
  int i;
};
}
class G {
  std::unique_ptr<const A> foo() const;
};
std::unique_ptr<const A> G::foo() const { return std::make_unique<const A>(); }

$ cat foo2.cpp
#include <memory>
namespace {
class A {
  bool a;
};
}
class H {
  std::unique_ptr<const A> bar() const;
};
std::unique_ptr<const A> H::bar() const { return std::make_unique<const A>(); }

$ /opt/gcc-5-branch/bin/g++ -std=c++14 -flto foo1.cpp foo2.cpp
/opt/gcc-5-branch/include/c++/5.2.1/bits/unique_ptr.h:151:41: warning: type
'const struct element_type' violates one definition rule [-Wodr]
       typedef _Tp                       element_type;
                                         ^
/opt/gcc-5-branch/include/c++/5.2.1/bits/unique_ptr.h:151:41: note: a different
type is defined in another translation unit
       typedef _Tp                       element_type;
                                         ^
foo1.cpp:4:7: note: the first difference of corresponding definitions is field
'i'
   int i;
       ^
foo2.cpp:4:8: note: a field with different name is defined in another
translation unit
   bool a;
        ^
/usr/lib/../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

whereas:

$ /opt/gcc-trunk/bin/g++ -flto -std=c++14 -flto foo1.cpp foo2.cpp
/usr/lib/../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status