Fix FM715.FOR

Character parameters were being added to the parse tree under the wrong
node type (although that didn't stop it emitting the correct code). The
main bug was that concat with constant parameters was evaluating to
CHAR type and CHAR with FIXEDCHAR promotes to CHAR by default. This
causes the .EQ. operator do to CHAR compare rather than FIXEDCHAR
compare. Now if FIXEDCHAR is on either side of a concat operator, the
result is always FIXEDCHAR.