[flang] Prevent RESULT name from being the same as the function.

Fix flang-compiler/f18#726.
Add test and error messsage for such scenario and set error on the
result symbol because it is unclear what it is inside the function.

Original-commit: flang-compiler/f18@a3d5b5af0d4a2537e2cd1cf84cae16e622d2c50c
Reviewed-on: https://github.com/flang-compiler/f18/pull/743
Tree-same-pre-rewrite: false