[flang][runtime] Treatment of NaN in MAXVAL/MAXLOC/MINVAL/MINLOC (#76999)

Detect NaN elements in data and handle them like gfortran does (at
runtime); namely, NaN can be returned if all the data are NaNs, but any
non-NaN value is preferable. Ensure that folding returns the same
results as runtime computation.

Fixes llvm-test-suite/Fortran/gfortran/regression/maxloc_2.f90 (and
probably others).