[GHC] #10913: deprecate and then remove -fwarn-hi-shadowing

[Chris Upshaw]

It seems like we lost the code that -fwarn-hi-shadow switched during the
add of hierarchical modules in 5.0. And while the hierarchical system makes
the common way .hi files could get shadowed impossible, you can still get
.hi file shadowing if you use the -i flag on multiple folders.

Do we care? It is a potential source of very hard to find bugs, but it is
also kind of a pathological case with how the modern module system works.
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