no-copy semantics for function forwarding

```zig
fn foo() Foo {
    return bar();
}
```

```llvm
define internal fastcc void @foo(%Foo* nonnull sret) unnamed_addr #2 !dbg !48 {
Entry:
  call fastcc void @bar(%Foo* sret %0), !dbg !52
  ret void, !dbg !54
}
```
ccce3d8526Andrew Kelley committed on 5/31/2019, 5:36:57 AMparent461382a
3 files changedLine totals unavailable