[flang] Changes to check for calls to PURE procedure components

This addresses issue flang-compiler/f18#781.  I changed my test to create a PURE procedure
component by using an interface as suggested by Peter and Jean.  I then
enhanced the function IsPureProcedure() in tools.cc to cover this case
and updated the code in check-do.cc to perform the test.

Original-commit: flang-compiler/f18@5df56a217dfc93775305ecb077aa4de2852ab71f
Reviewed-on: https://github.com/flang-compiler/f18/pull/789