Avoid producing mismatched comdats.

The problem was that given

template<typename T>
struct foo {
  ~foo() {}
};
template class foo<int>;

We would produce a alias, creating a comdat with D0 and D1, since the symbols
have to be weak. Another TU is not required to have a explicit template
instantiation definition or an explict template instantiation declaration and
for

template<typename T>
struct foo {
  ~foo() {}
};
foo<int> a;

we would produce a comdat with only one symbol in it.

llvm-svn: 194520