[flang][TBAABuilder] not all loads and stores are inside of functions

TBAA builder assumed that all loads/stores are inside of functions and
hit an assertion once it found loads and stores inside of an
omp::ReductionDeclareOp.

For now just don't add TBAA tags to those loads and stores. They would
end up in a different TBAA tree to the host function after
OpenMPIRBuilder inlines them anyway so there isn't an easy way of making
this work.

Commit series for by-ref OpenMP reductions: 2/3