Avoid stack overflow in triangular eigvecs (#55497)

This fixes a stack overflow in 
```julia
julia> using LinearAlgebra, StaticArrays

julia> U = UpperTriangular(SMatrix{2,2}(1:4))
2×2 UpperTriangular{Int64, SMatrix{2, 2, Int64, 4}} with indices SOneTo(2)×SOneTo(2):
 1  3
 ⋅  4

julia> eigvecs(U)
Warning: detected a stack overflow; program state may be corrupted, so further execution might be unreliable.
ERROR: StackOverflowError:
Stacktrace:
 [1] eigvecs(A::UpperTriangular{Float32, SMatrix{2, 2, Float32, 4}}) (repeats 79984 times)
   @ LinearAlgebra ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/LinearAlgebra/src/triangular.jl:2749
```
After this,
```julia
julia> eigvecs(U)
2×2 Matrix{Float32}:
 1.0  1.0
 0.0  1.0
```

stdlib/LinearAlgebra/src/triangular.jl

@@ -2782,6 +2782,14 @@ end
 
 # Generic eigensystems
 eigvals(A::AbstractTriangular) = diag(A)
+# fallback for unknown types
+function eigvecs(A::AbstractTriangular{<:BlasFloat})
+    if istriu(A)
+        eigvecs(UpperTriangular(Matrix(A)))
+    else # istril(A)
+        eigvecs(LowerTriangular(Matrix(A)))
+    end
+end
 function eigvecs(A::AbstractTriangular{T}) where T
     TT = promote_type(T, Float32)
     if TT <: BlasFloat

stdlib/LinearAlgebra/test/triangular.jl

@@ -1198,6 +1198,22 @@ end
     end
 end
 
+@testset "eigvecs for AbstractTriangular" begin
+    S = SizedArrays.SizedArray{(3,3)}(reshape(1:9,3,3))
+    for T in (UpperTriangular, UnitUpperTriangular,
+                LowerTriangular, UnitLowerTriangular)
+        U = T(S)
+        V = eigvecs(U)
+        λ = eigvals(U)
+        @test U * V ≈ V * Diagonal(λ)
+
+        MU = MyTriangular(U)
+        V = eigvecs(U)
+        λ = eigvals(U)
+        @test MU * V ≈ V * Diagonal(λ)
+    end
+end
+
 @testset "(l/r)mul! and (l/r)div! for generic triangular" begin
     @testset for T in (UpperTriangular, LowerTriangular, UnitUpperTriangular, UnitLowerTriangular)
         M = MyTriangular(T(rand(4,4)))