improve inference in LinearAlgebra/symmetric

[simeonschaub]

should fix #356

[simeonschaub]

The remaining two failures seem to be Julia bugs (ref JuliaLang/julia#39641). Would be good to keep an eye on whether this always affects the same platforms and report that back.

[codecov-io]

Codecov Report

Merging #371 (fb311af) into master (adcaa91) will decrease coverage by 0.05%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   97.78%   97.72%   -0.06%     
==========================================
  Files          19       19              
  Lines        1487     1495       +8     
==========================================
+ Hits         1454     1461       +7     
- Misses         33       34       +1     

Impacted Files	Coverage Δ
src/rulesets/LinearAlgebra/symmetric.jl	98.32% <96.15%> (-0.38%)	⬇️
src/rulesets/LinearAlgebra/factorization.jl	97.68% <100.00%> (ø)

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