[flang] Ensure a characterized ENTRY in a PURE subprogram is also marked PURE

ENTRY point symbols aren't marked PURE in the symbol table, but must
instead inherit the attribute from their containing subprograms.
There's a predicate in semantics that does this, but it wasn't being
used in the context of actual procedure argument characterization.

Differential Revision: https://reviews.llvm.org/D119564