Some additional cleanups with method names.

Renamed ExprValue to RValue, as all expression values are RValues, and this
keeps with the C terminology (renamed old "RValue" class to "NonLValue").

Introduced "ConcreteInt", a class that represents a concrete, integer
constant as an RValue.

Temporarily removed classes to represent set of possible constants and set of !=
constants. Will replace with a more general class representing a set of
constraints.

Added some foundational code to track "symbolic" values, which are used to
accrue constraints on an abstract value that is shared between multiple
variables.  e.g:

   x = y;      // at this point "x" and "y" share the same "value"
   if (x > 1) 
     ...       // at this point, the value shared by "x" and "y" is "> 1".

llvm-svn: 46466