Rename HIR UnOp variants

[osa1]

This renames the variants in HIR UnOp from

enum UnOp {
    UnDeref,
    UnNot,
    UnNeg,
}

to

enum UnOp {
    Deref,
    Not,
    Neg,
}

Motivations:

• This is more consistent with the rest of the code base where most enum
  variants don't have a prefix.

• These variants are never used without the UnOp prefix so the extra
  Un prefix doesn't help with readability. E.g. we don't have any
  UnDerefs in the code, we only have UnOp::UnDeref.

• MIR UnOp type variants don't have a prefix so this is more
  consistent with MIR types.

• "un" prefix reads like "inverse" or "reverse", so as a beginner in
  rustc code base when I see "UnDeref" what comes to my mind is
  something like &* instead of just *.

[rust-highfive]

r? @matthewjasper

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[matthewjasper]

@bors r+

[bors]

📌 Commit c4e3558 has been approved by matthewjasper