Functions in ConstantFold.h were declared as internal interface so this
header resided in lib directory. However the functions declared there
are useful in other places, in particular, in ConstantFolding.cpp. To
avoid code duplication, these functions are moved into include
directory, so the interface declared in ConstantFold.h is not internal
This change only modes the include file, fixes references to it and
removes mentions about internal interface. No functional changes.