Use intermOut.cpp's IsNan and IsInfinity for parse-time constant folding

Use intermOut.cpp's IsNan and IsInfinity for parse-time constant folding

There were two implementations of isInf() and isNan(), in Constant.cpp
and in intermOut.cpp. The former only works on little-endian systems,
the latter is a wrapper for library functions and works regardless of
endianness. Move the second version into Common.h and adopt it in both
places. Thereby avoid the duplication and fix for big-endian systems.

On s390x, this fixes the test case
Glsl/CompileToAstTest.FromFile/constFold_frag.

Fixes #2802