[flang] Don't allow non-standard data conversions of potentially abse… (#87391)

…nt arguments

Arguments to the intrinsic functions MAX and MIN after the first two are
optional. When these actual arguments might not be present at run time,
emit a compilation time error if they require data conversion (a
non-standard but nearly universal language extension); such a conversion
would crash if the argument was absent.

Other compilers either disallow data conversions entirely on MAX/MIN or
crash at run time if a converted argument is absent.

Fixes https://github.com/llvm/llvm-project/issues/87046.