Add NaN and Inf defaults to the C++ generated code. (#5102)

* Add `NaN` and `Inf` defaults to the C++ generated code.

* Refactoring: add FloatConstantGenerator

* Refactoring-2:

- remove isnan checking for all float/double values
- add most probable implementation of virtual methods of FloatConstantGenerator

* Add conditional (FLATBUFFERS_NAN_DEFAULTS) isnan checking

tests/monster_extra.fbs

@@ -0,0 +1,12 @@
+namespace MyGame;
+
+// Not all programmining languages support this extra table.
+table MonsterExra {
+  // Float-point values with NaN and Inf defaults.
+  testf_nan:float = nan;
+  testf_pinf:float = +inf;
+  testf_ninf:float = -inf;
+  testd_nan:double = nan;
+  testd_pinf:double = +inf;
+  testd_ninf:double = -inf;
+}