[flang] Fix PURE check on procedure binding

A symbol that represents a procedure binding is PURE if the procedure
it is bound to is PURE. Fix `IsPureProcedure` to check that.

Make use of `IsPureProcedure` in `CheckSpecificationExprHelper`.

Original-commit: flang-compiler/f18@c95f2eb4fb25d464977690ba03260be2451911d7
Reviewed-on: https://github.com/flang-compiler/f18/pull/849