rename is_lvalue -> is_assignable

[kroening]

The term 'lvalue' is nomenclature that is specific to C/C++. The function
is_lvalue is meant to be used in the context of goto-programs, and thus, a
name that suggests a relationship to C programs may be confusing.

• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6378 (b29a6db) into develop (54a50e9) will decrease coverage by 0.00%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           develop    #6378      +/-   ##
===========================================
- Coverage    75.91%   75.91%   -0.01%     
===========================================
  Files         1517     1517              
  Lines       164024   164022       -2     
===========================================
- Hits        124515   124513       -2     
  Misses       39509    39509              

Impacted Files	Coverage Δ
src/util/cmdline.h	66.66% <ø> (ø)
src/util/expr_util.h	100.00% <ø> (ø)
src/util/expr_util.cpp	88.15% <66.66%> (ø)
src/goto-programs/builtin_functions.cpp	55.32% <75.00%> (ø)
...yses/variable-sensitivity/abstract_environment.cpp	95.02% <100.00%> (ø)
src/util/cmdline.cpp	95.32% <100.00%> (-0.06%)	⬇️
src/util/replace_symbol.cpp	87.57% <100.00%> (ø)

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[tautschnig]

Should this perhaps be moved to goto-programs? The use in replace_symbol obviously conflicts with that, but some follow-up cleanup may be necessary.

[kroening]

Yes; there may well be a need to make copies of this function for particular uses. I'd see this as an orthogonal question to the renaming done here.