[flang] Add -fsyntax-only to f18; retain -fparse-only synonym

Now that semantics is working, the standard -fsyntax-only option of
GNU and Clang should be used as the name of the option that causes
f18 to just run the front-end.  Support both options in f18, silently
accepting the old option as a synonym for the new one (as
preferred by the code owner), and replace all instances of the
old -fparse-only option with -fsyntax-only throughout the source base.

Differential Revision: https://reviews.llvm.org/D95887