[DebugInfo][NFC] Rename the class DbgValueLocation to DbgVariableValue

Rename the class DbgValueLocation to DbgVariableValue and instances from Loc to
DbgValue. These names better express the new semantics introduced in D74053.

The class previously represented a { Location } only. It now represents a
{ Location, DIExpression } pair which together describe a value.

Reviewed By: aprantl
Differential Revision: https://reviews.llvm.org/D74055