Add for_each_predecessor() to grapht

[johnfxgalea]

Adds for_each_predecessor() to grapht, along with some tests.

• Each commit message has a non-empty body, explaining why the change was made.
• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6160 (538070c) into develop (35a5f17) will increase coverage by 0.03%.
The diff coverage is 95.23%.

@@             Coverage Diff             @@
##           develop    #6160      +/-   ##
===========================================
+ Coverage    74.92%   74.95%   +0.03%     
===========================================
  Files         1450     1450              
  Lines       158265   158274       +9     
===========================================
+ Hits        118575   118632      +57     
+ Misses       39690    39642      -48     

Impacted Files	Coverage Δ
unit/util/graph.cpp	99.30% <95.00%> (-0.70%)	⬇️
src/util/graph.h	97.29% <100.00%> (+<0.01%)	⬆️
src/ansi-c/ansi_c_entry_point.cpp	66.04% <0.00%> (-21.40%)	⬇️
src/ansi-c/c_typecheck_type.cpp	63.54% <0.00%> (-12.85%)	⬇️
src/util/json.cpp	85.58% <0.00%> (-1.81%)	⬇️
src/util/rename_symbol.cpp	79.09% <0.00%> (+1.81%)	⬆️
src/ansi-c/c_typecast.cpp	79.17% <0.00%> (+3.56%)	⬆️
src/util/validate_code.cpp	85.00% <0.00%> (+5.00%)	⬆️
src/ansi-c/type2name.cpp	89.94% <0.00%> (+5.02%)	⬆️
... and 5 more

---

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 26f9de7...538070c. Read the comment docs.