[flang] Answer PR 631 comments:

 - Create a function `IsFunctionResult(const Symbol &)` in
   lib/semantics/tools.h.
 - style edit in weird var definition style.

Original-commit: flang-compiler/f18@15e3f87b40345765ac500e5f8c1412c31d4e2658
Reviewed-on: https://github.com/flang-compiler/f18/pull/631
Tree-same-pre-rewrite: false