This deletes some legacy cruft, and produces leaner object files.
Example:

```
var x: i32 = 1234;

export fn entry() i32 {
    return x;
}
```

This produces:

```
@x = internal unnamed_addr global i32 1234, align 4
@0 = internal unnamed_addr constant i32* @x, align 8
```

and @0 is never even used. After this commit, @0 is not produced.

This fixes a bug: Zig was creating invalid LLVM IR when one of these
globals that shouldn't exist takes the address of a thread local
variable. In LLVM 8.0.0rc2, it would produce a linker error. But
probably after my bug report is solved it will be caught by the IR
verifier.

https://bugs.llvm.org/show_bug.cgi?id=40652