Rename RK_const_value to RK_expr_result

[katzdm]

P2996R3 drops "reflections of expressions", and makes it clear that what's reflected is a "result of evaluating an expression" - which can either be a value (if evaluated from a prvalue) or an object (if evaluated from an lvalue). As a bit of cleanup, it'd be nice to everywhere change the nomenclature from RK_const_value (and getReflectedConstValueExpr, etc) to RK_expr_result.

Still not clear whether storing this in a ConstantExpr was the smartest decision, but it doesn't seem to be getting in the way as of yet.