[lldb] Remove 'use_synthetic' parameters in ValueObject code

Summary:
`CalculateSyntheticValue` and `GetSyntheticValue` have a `use_synthetic` parameter
that makes the function do nothing when it's false. We obviously always pass true
to the function (or check that the value we pass is true), because there really isn't
any point calling with function with a `false`. This just removes all of this.

Reviewers: labath, JDevlieghere, davide

Reviewed By: davide

Subscribers: davide

Differential Revision: https://reviews.llvm.org/D79568