[flang] Address comment: make error a warning instead and ignore RESULT

Instead of emitting an error when RESULT has the same name as the
function, emit a warning and ignore RESULT in the rest of the
compilation.

Original-commit: flang-compiler/f18@8ce3862d39cee904b719ed68752237b20d9e154f
Reviewed-on: https://github.com/flang-compiler/f18/pull/743