properly annotate nullability of ZigLLVMCreateDebugForwardDeclType

This bug manifested as a segfault in stage1 when calling this function.
The C++ code looks like this:

```c++
entry->llvm_di_type = ZigLLVMCreateDebugForwardDeclType(g->dbuilder,
    ZigLLVMTag_DW_structure_type(), full_name,
    import ? ZigLLVMFileToScope(import->data.structure.root_struct->di_file) : nullptr,
    import ? import->data.structure.root_struct->di_file : nullptr,
    line);
```

There is actually no problem here - what happened is that because
cross-language LTO was enabled between zig and c++ code, and because
Zig annotated the file parameter (3rd line) as being non-null, the C++
code assumed that parameter could not be null, and eagerly dereferenced
`import->...`, causing a segfault, since it was null.

I verified that this commit fixed the problem and I also verified this
hypothesis by disabling LTO and noticing that it indeed avoided the
problem.

src/codegen/llvm/bindings.zig

@@ -1734,8 +1734,8 @@ pub const DIBuilder = opaque {
         dib: *DIBuilder,
         tag: c_uint,
         name: [*:0]const u8,
-        scope: *DIScope,
-        file: *DIFile,
+        scope: ?*DIScope,
+        file: ?*DIFile,
         line: c_uint,
     ) *DIType;