[msl-out] Make varyings' struct members unique

[evahop]

Fixes #1490

Naming conflicts can occur when multiple structs get flattened into one varyings struct.
This moves the namer.call invocation into the flattening routine so that each member gets a unique name.

[teoxoy]

Thanks for the PR! It looks good but could you add a test for this? The one in the issue would work.

[evahop]

Yes, I was going to add that there's also a namer.namespace method specifically for structs, It may be more appropriate, but I am not familiar enough with result types. I will try to use that instead, but no promises. I didn’t bother with it initially because I wasn’t sure if I’d be able to access those new names anyhow.

[evahop]

Added the snapshot, which also confirms #1464 has been fixed (not by me).
Abandoned namer.namespace and ended up just adding a new namer solely for the locations.

[teoxoy]

Seems like #1464 was fixed by #1766.

[teoxoy]

Good stuff!