Suggested improvements to `usage-of-qualified-ty` lint

[alexreg]

Firstly, I don't even know why ty::Ty is a big problem... but I guess it's a readability choice someone made? Moreover, these lints had me stumped for a while.

• "Qualified" should probably read something like "Namespace-qualified" to be explicit.
• Ty<> should read Ty.
• The spans aren't great here. (They should probably just point to the Ty::<'tcx> bits.)

error: usage of qualified `ty::Ty<'tcx>`
   --> src/librustc_mir/interpret/operand.rs:175:18
    |
175 |         let ty = ty::Ty::<'tcx>::decode(self)?;
    |                  ^^^^^^^^^^^^^^^^^^^^^^ help: try using it unqualified: `Ty<'tcx>`
    |
    = note: `-D rustc::usage-of-qualified-ty` implied by `-D rustc::internal`

error: usage of qualified `ty::Ty<>`
   --> src/librustc_mir/interpret/operand.rs:175:18
    |
175 |         let ty = ty::Ty::decode(self)?;
    |                  ^^^^^^^^^^^^^^ help: try using it unqualified: `Ty<>`
    |
    = note: `-D rustc::usage-of-qualified-ty` implied by `-D rustc::internal`

CC @estebank

[Centril]

"try using it unqualified: Ty<>" is just wrong at any rate due to lack of lifetimes; should be Ty<'_>.

[flip1995]

I must admit, that I didn't put much effort in the suggestion and spans of the internal lints, since they're "just internal". I'm looking into improving those in the near future.

[alexreg]

@flip1995 Which is fair enough really. Improving them would be welcome though yes.