Refactor c_typecheck_baset::do_initializer

[tautschnig]

This function had (almost) the same code across several branches. The
refactoring reduces the number of lines of code and removes an "assert"
statement: make_constant will report an error if the result isn't a
constant, making this assertion unnecessary.

• 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.

[codecov]

Codecov Report

Merging #6660 (feff062) into develop (52d5788) will decrease coverage by 0.00%.
The diff coverage is 68.18%.

@@             Coverage Diff             @@
##           develop    #6660      +/-   ##
===========================================
- Coverage    76.81%   76.81%   -0.01%     
===========================================
  Files         1582     1582              
  Lines       182764   182764              
===========================================
- Hits        140399   140397       -2     
- Misses       42365    42367       +2     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_expr.cpp	76.45% <50.00%> (-0.15%)	⬇️
src/ansi-c/c_typecheck_initializer.cpp	75.18% <90.00%> (+0.18%)	⬆️

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