[flang] Fine-tune NAN formatted input editing

Per Fortran 2018, "NAN" and "NAN()" are to be translated into quiet
NaNs, and the other forms are implementation-dependent;  I've made
them quiet NaNs too.  Also process signs on input NaNs, which seems
wrong but other compilers all do it, and fix some misleading template
argument names noticed along the way.

Differential Revision: https://reviews.llvm.org/D147071