IR: Add ConstantData, for operand-less Constants

Add a common parent `ConstantData` to the constants that have no
operands.  These are guaranteed to represent abstract data that is in no
way tied to a specific Module.

This is a good cleanup on its own.  It also makes it simpler to disallow
RAUW (and factor away use-lists) on these constants in the future.  (I
have some experimental patches that make RAUW illegal on ConstantData,
and they seem to catch a bunch of bugs...)

llvm-svn: 261464