Aggressive constprop in trevc! to stabilize triangular eigvec (#54635)

We may use aggressive constprop in `trevc!` to eliminate branches, which
makes the return type of `eigvec(::UpperTriangular)` concretely
inferred.
```julia
julia> @inferred eigvecs(UpperTriangular([1 0; 0 1]))
2×2 Matrix{Float32}:
 1.0  -0.0
 0.0   1.0
```

(cherry picked from commit ea8b4af)

stdlib/LinearAlgebra/src/lapack.jl

@@ -3633,7 +3633,7 @@ for (trcon, trevc, trrfs, elty) in
         # LOGICAL            SELECT( * )
         # DOUBLE PRECISION   T( LDT, * ), VL( LDVL, * ), VR( LDVR, * ),
         #$                   WORK( * )
-        function trevc!(side::AbstractChar, howmny::AbstractChar, select::AbstractVector{BlasInt}, T::AbstractMatrix{$elty},
+        Base.@constprop :aggressive function trevc!(side::AbstractChar, howmny::AbstractChar, select::AbstractVector{BlasInt}, T::AbstractMatrix{$elty},
                         VL::AbstractMatrix{$elty} = similar(T),
                         VR::AbstractMatrix{$elty} = similar(T))
             require_one_based_indexing(select, T, VL, VR)

stdlib/LinearAlgebra/test/triangular.jl

@@ -879,4 +879,10 @@ end
     @test transpose!(U) == Ut
 end
 
+@testset "type-stable eigvecs" begin
+    D = Float64[1 0; 0 2]
+    V = @inferred eigvecs(UpperTriangular(D))
+    @test V == Diagonal([1, 1])
+end
+
 end # module TestTriangular