[flang] Changes to improve messages for DO semantics

I changed two things in the messages produced for DO semantic checks --
 - I changed the source location to be specific to where conflicting names are
 declared.
 - I added the conflicting names to the message.

Original-commit: flang-compiler/f18@f39f9ceeae3a6471eae2a4e651d39a0921416b1b
Reviewed-on: https://github.com/flang-compiler/f18/pull/510