[DebugInfo][NFC] Fixup the UserValue methods to use FragmentInfo

Fixup the UserValue methods to use FragmentInfo instead of DIExpression because
the DIExpression is only ever used to get the to get the FragmentInfo. The
DIExpression is meaningless in the UserValue class because each definition point
added to a UserValue may have a unique DIExpression.

Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D74057