avoid conversion from Char to String to Symbol in some LinearAlgebra routines

[KristofferC]

This round-tripping between symbols and chars everywhere feels non-ideal but anyway.

Benchmark

using LinearAlgebra
using BenchmarkTools
a = Symmetric(rand(10,10));
@btime $(a)[1,1]

Before PR:

  105.620 ns (1 allocation: 32 bytes)

After PR

  2.688 ns (0 allocations: 0 bytes)

[KristofferC]

This regressed vs 0.6, seems introduced in #25688

[Sacha0]

Perhaps regrettable in terms of readability, but otherwise lgtm! :)

[fredrikekre]

backport?

[KristofferC]

This regression. Is pretty bad so I think for 1.0.2 it would be good to have.

[oscardssmith]

Wait, why are strings is Chars involved here?

[KristofferC]

@edit Symbol('U')