isCopyConstructable should call isCopyable

[WalterBright]

Seeing if this can be done.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16944"

[WalterBright]

This fails to compile libmir with:

source/mir/rc/ptr.d(395,15): Error: `mir.rc.ptr.castTo` called with argument types `(mir_rcptr!(C))` matches both:
  source/mir/rc/ptr.d(275,13):     `mir.rc.ptr.castTo!(I, C).castTo(mir_rcptr!(C) context)`
  and:
  source/mir/rc/ptr.d(291,25):     `mir.rc.ptr.castTo!(I, C).castTo(immutable(mir_rcptr!(C)) context)`