add source tracking in VCGenerator

[konnov]

From time to time VCGenerator is producing intermediate expressions that are not tracked. As a result, we get a bunch of diagnostic messages like:

No source location for expr@2656122: ∃n6f1b1_inductive::inductive::commit_qcs$1 ∈ ([(0 .. 4) → ({LET n6f1b...
No source location for expr@2657115: (LET n6f1b1_inductive::inductive::init_timeout_qc$1 ≜ ["votes" ↦ ([n6... 

This PR fixes source tracking in VCGenerator.

• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Entries added to ./unreleased/ for any new functionality

[thpani]

Amazing!