Constant propagation of deleteCharAt()

[danpoe]

This implements constant propagation of CProverString.deleteCharAt() which in turn enables constant propagation of StringBuilder.deleteCharAt() and StringBuffer.deleteCharAt().

• 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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 193a29b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/128832294

[codecov-io]

Codecov Report

Merging #5126 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #5126      +/-   ##
===========================================
+ Coverage    66.96%   66.96%   +<.01%     
===========================================
  Files         1146     1145       -1     
  Lines        93711    93687      -24     
===========================================
- Hits         62752    62737      -15     
+ Misses       30959    30950       -9

Flag	Coverage Δ
#cproversmt2	42.66% <0%> (-0.02%)	⬇️
#regression	63.46% <100%> (ø)	⬆️
#unit	31.82% <0%> (-0.1%)	⬇️

Impacted Files	Coverage Δ
src/goto-symex/goto_symex.h	92.3% <ø> (ø)	⬆️
src/goto-symex/goto_symex.cpp	99.5% <100%> (+0.06%)	⬆️
...bmc/src/java_bytecode/java_static_initializers.cpp	99.09% <0%> (-0.03%)	⬇️
jbmc/src/java_bytecode/java_bytecode_language.h	76.92% <0%> (ø)	⬆️
jbmc/src/java_bytecode/code_with_references.cpp
jbmc/src/java_bytecode/code_with_references.h
jbmc/src/java_bytecode/assignments_from_json.h	100% <0%> (ø)
jbmc/src/java_bytecode/assignments_from_json.cpp	98.27% <0%> (+0.27%)	⬆️
src/solvers/prop/prop_conv_solver.cpp	81.33% <0%> (+0.44%)	⬆️
src/util/std_code.h	92.43% <0%> (+0.48%)	⬆️

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