[flang] Cleanup of NYI messages (#73740)

This update makes the user visible messages relating to features that
are not yet implemented be more consistent. I also cleaned up some of
the code.

For NYI messages that refer to intrinsics, I made sure the the message
begins with "not yet implemented: intrinsic:" to make them easier to
recognize.

I created some utility functions for NYI reporting that I put into
.../include/Optimizer/Support/Utils.h. These mainly convert MLIR types
to their Fortran equivalents.

I converted the NYI code to use the newly created utility functions.