fix dispatch for schur(::UpperHessenberg)

[stevengj]

Fixes an apparent bug in JuliaLang/julia#47872 — schur(A::UpperHessenberg{T}) called copy_similar(A, eigtype(T))) but the copy_similar function strips away the UpperHessenberg wrapper. As a result, the optimized schur! function was not being called.

Following #1557, this changes it to call eigencopy_oftype, which preserves the UpperHessenberg wrapper (and also Hermitian wrappers, but schur already has a specialized dispatch for Hermitian cases). It also adds a fallback schur! for non-BLAS types, to avoid breaking anything.

cc @albertomercurio, @Jutho, and @dkarrasch, who were involved in the original PR.

[stevengj]

It's not clear to me why this schur(A::UpperHessenberg{T}) method was added in the first place — it seems redundant with schur(A::AbstractMatrix{T})?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (bbaf1f6) to head (fa191cb).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1565      +/-   ##
==========================================
- Coverage   94.35%   94.32%   -0.03%     
==========================================
  Files          35       35              
  Lines       16004    16004              
==========================================
- Hits        15100    15096       -4     
- Misses        904      908       +4     

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