[flang] Accept sparse argument keyword names for MAX/MIN

Accept any keyword argument names of the form "An" for
values of n >= 3 in calls to the intrinsic functions MAX, MIN,
and their variants, so long as "n" has no leading zero and
all the keywords are distinct.  Previously, f18 was needlessly
requiring the names to be contiguous.  When synthesizing keywords
to characterize the procedure's interface, don't conflict with
the program's keywords.

Differential Revision: https://reviews.llvm.org/D117701