[flang] Add nonfatal message classes

F18 presently has fatal and non-fatal diagnostic messages.  We'd like
to make non-fatal warnings stand out better in the output of the compiler.

This will turn out to be a large change that affects many files.
This patch is just the first part.  It converts a Boolean isFatal_ data
member of the message classes into a severity code, and defines four
of these codes (Error, Warning, Portability, and a catch-all Other).

Later patches will result from sweeping over the parser and semantics,
changing most non-fatal diagnostic messages into warnings and portability
notes.

Differential Revision: https://reviews.llvm.org/D121228