[Flang] NFC: Changes to adhere to coding guidelines

This patch includes some changes which brings the code in line with
llvm coding guidelines.
-> Remove curlies for one line if statements.
-> Remove else after return.
-> Removes a few usage of auto.
-> Add Doxygen comments

Addresses post review comments in D120403 by @schweitz.

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D120657