[PDB] `findFullDeclForForwardRef` can return incorrect type index in incrementally linked PDBs

Incrementally linked PDBs can contain more than one tag type (`LF_CLASS`, `LF_STRUCTURE`, `LF_UNION`) with the same unique name and thus the same hash value. When encountering a forward reference to such type, it's not clear which type is referenced. Consider the following example:

1. Create `main.cpp`
```cpp
struct Foo {
// Foo *ptr = nullptr;
int a = 1;
};

int main() {
Foo f;
Foo *p = &f;
return f.a + p->a;
}
```
2. Compile with
```
cl main.cpp /GS- /Z7 /nologo /link /nodefaultlib /entry:main
```
3. Uncomment `Foo *ptr = nullptr;`
4. Recompile

When we dump the types (TPI) from generated PDB, we get

- 0x1000: LF_ARGLIST (...)
- 0x1001: LF_PROCEDURE (...)
- 0x1002: LF_STRUCTURE name: `Foo`, unique name: `.?AUFoo@@`, options: forward ref | has unique name
- 0x1003: LF_POINTER referent: 0x1002
- 0x1004: LF_MFUNCTION (...)
- 0x1005: LF_FIELDLIST { a, Foo }
- 0x1006: LF_STRUCTURE name: `Foo`, unique name: `.?AUFoo@@`, field list: 0x1005, options: has ctor / dtor | has unique name
- 0x1007: LF_FIELDLIST { ptr, a, Foo }
- 0x1008: LF_STRUCTURE name: `Foo`, unique name: `.?AUFoo@@`, field list: 0x1007, options: has ctor / dtor | has unique name

**Which type is 0x1002 referring to?**
From the program, we know that it must refer to 0x1008, because that type has the `ptr` member.
But how could we know this from the PDB?

Currently, [`TpiStream::findFullDeclForForwardRef`](https://github.com/llvm/llvm-project/blob/f146677396ea0b02b902743633a1f934023ae15f/llvm/lib/DebugInfo/PDB/Native/TpiStream.cpp#L178-L218) hashes the unique name and looks in the hash table for a non-forward-reference. This is used to annotate forward references in `llvm-pdbutil dump`.
However, in our case, we will find two entries. It picks the first, which is incorrect.
While it's correct here, always picking the last entry isn't a solution either. If we comment out `Foo *ptr = nullptr;` again and recompile, we will find that the records in the TPI stream haven't changed. So now, picking the first entry would be correct.

LLDB's native PDB reader uses `findFullDeclForForwardRef` as well, so it will sometimes show the wrong type.

I'm not sure what the correct approach here is. Both MS DIA and the VS debugger resolve the correct type.