[flang] Fix issue flang-compiler/f18#486

In a function without a function result specified, the name in the
`FunctionStmt` was resolved to the function result symbol rather than
the function symbol itself. That is inconsistent with subroutines and
other functions.

The fix is to explicitly set the function name symbol after the result
has been resolved.

Original-commit: flang-compiler/f18@b0172d32848b6135f076b3171ea55ea6f63de564
Reviewed-on: https://github.com/flang-compiler/f18/pull/487