add missing source_location for assignments generated for initializers

[kroening]

A variable definition in C may include an initializer. We split these into
a DECL instruction and an ASSIGN instruction. This commit ensures that the
ASSIGN instruction has a source_location attached to it (clean_expr may
yield a temporary, which does not appear in the source code).

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

The failing tests appear to demonstrate the (desired) effect, but will still need fixing.

[codecov]

Codecov Report

Merging #6880 (5e2c337) into develop (79169e0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 5e2c337 differs from pull request most recent head bcca9da. Consider uploading reports for the commit bcca9da to get more accurate results

@@           Coverage Diff            @@
##           develop    #6880   +/-   ##
========================================
  Coverage    77.80%   77.80%           
========================================
  Files         1567     1567           
  Lines       179915   179916    +1     
========================================
+ Hits        139987   139988    +1     
  Misses       39928    39928           

Impacted Files	Coverage Δ
src/goto-programs/goto_convert.cpp	92.34% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 679ab3f...bcca9da. Read the comment docs.

[tautschnig]

It seems that location15 also needs updating.