c++: Fix NaN as C++20 template argument

C++20 allows floating-point types for non-type template parameters;
floating-point values are considered to be equivalent template arguments if
they are "identical", which conveniently seems to map onto an existing GCC
predicate.

gcc/cp/ChangeLog:

	* tree.c (cp_tree_equal): Use real_identical.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-float1.C: New test.