[flang][driver] Move isFixedFormSuffix and isFreeFormSuffix to flangFrontend

isFixedFormSuffix and isFreeFormSuffix should be defined in
flangFrontend rather than flangFrontendTool library. That's for 2
reasons:
  * these methods are used in flangFrontend rather than flangFrontendTool
  * flangFrontendTool depends on flangFrontend

As mentioned in the post-commit review for D94228, without this change
shared library builds fail.

Differential Revision: https://reviews.llvm.org/D94968