[flang] IEEE_NEXT_AFTER, IEEE_NEXT_DOWN, IEEE_NEXT_UP, NEAREST (#100782)

IEEE_ARITHMETIC intrinsic module procedures IEEE_NEXT_AFTER,
IEEE_NEXT_DOWN, and IEEE_NEXT_UP, and intrinsic NEAREST return larger or
smaller values adjacent to their primary REAL argument. The four
procedures vary in how the direction is chosen, in how special cases are
treated, and in what exceptions are generated. Implement the three
IEEE_ARITHMETIC procedures. Update the NEAREST implementation to support
all six REAL kinds 2,3,4,8,10,16, and fix several bugs.

IEEE_NEXT_AFTER(X,Y) returns a NaN when Y is a NaN as that seems to be
the universal choice of other compilers.

Change the front end compile time implementation of these procedures to
return normal (HUGE) values for infinities when applicable, rather than
always returning the input infinity.

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