-ohi and -o don't behave consistently with -dynamic-too

-o has some special logic to also rename the dynamic output based on the argument to -o. There isn't the same logic for -ohi which leads to interface files getting overwritten.

The result if you use -ohi is that the dynamic interface file clobbers the normal interface file, both get written to the same path.

> ghc -c -o A2.o -ohi A2.hi A.hs -dynamic-too -fforce-recomp
> [matt@nixos:~/ghc-clean/dyn-dyn-too]$ ls
A2.dyn_o  A2.hi  A2.o  A.hs
> [matt@nixos:~/ghc-clean/dyn-dyn-too]$ ghc --show-iface A2.hi 
Magic: Wanted 33214052,
       got    33214052
Version: Wanted 8104,
         got    8104
Way: Wanted [],
     got    [d, y, n]

With head there is the -dynohi flag which should be used to be able to redirect the dynamic interface file (but that panics)

Proposal: Specifying -ohi also sets -dynohi if that's not set like -o sets -dyno.