strengthen type of parameter of gather_field_types

[kroening]

The first parameter of ci_lazy_methods_neededt::gather_field_types is always
a struct_tag_typet. This is now reflected in the signature.

• 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.
• n/a 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).
• n/a 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.

[smowton]

What makes you sure the tag was not already expanded before calling the method?

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 813efa3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/96898981

[kroening]

Well, we shouldn't expand!

[TGWDB]

Closing due to age (no further comment on PR content), please reopen with rebase on develop if you intent to continue this work.

[tautschnig]

Rebased and re-opened for this still seems to make sense.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bce472f) 78.51% compared to head (86640b4) 78.52%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3742   +/-   ##
========================================
  Coverage    78.51%   78.52%           
========================================
  Files         1674     1674           
  Lines       191946   191945    -1     
========================================
+ Hits        150715   150718    +3     
+ Misses       41231    41227    -4     

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/ci_lazy_methods_needed.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods_needed.cpp	98.38% <100.00%> (-0.03%)	⬇️

... and 1 file with indirect coverage changes

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