Made the IRInterpreter be able to operate without
a ClangExpressionDeclMap.  Any functions that
require value resolution etc. fail if the
ClangExpressionDeclMap isn't present - which is
exactly what is desired.
Don't store AttributeSet::FunctionIndex as an int.

GCC complains: Core.cpp:1449:27: warning: overflow in implicit constant conversion [-Woverflow]
I'm not sure if that's really a problem here, but using the enum type is better
style anyways.