[flang] Add IsElementalProcedure() predicate

Replace most tests of the explicit Attr::ELEMENTAL symbol flag with
a new predicate IsElementalProcedure() that works correctly for alternate
ENTRY points and does the right thing for procedure interfaces that
reference elemental intrinsic functions like SIN() whose elemental
nature does not propagate.

Differential Revision: https://reviews.llvm.org/D129022