Static-lifetime symbols require compile-time constant initialisers

[tautschnig]

We previously silently accepted initialisers that weren't compile-time
constants, resulting in surprising behaviour: the assertion in the
enclosed regression test would fail, because x ended up being
initialised before y.

• 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 #6461 (bc9627d) into develop (bfa4c2c) will decrease coverage by 0.05%.
The diff coverage is 65.70%.

❗ Current head bc9627d differs from pull request most recent head 1a86739. Consider uploading reports for the commit 1a86739 to get more accurate results

@@             Coverage Diff             @@
##           develop    #6461      +/-   ##
===========================================
- Coverage    76.01%   75.96%   -0.06%     
===========================================
  Files         1527     1542      +15     
  Lines       164465   165032     +567     
===========================================
+ Hits        125013   125360     +347     
- Misses       39452    39672     +220     

Impacted Files	Coverage Δ
src/crangler/ctokenit.cpp	0.00% <0.00%> (ø)
src/crangler/ctokenit.h	0.00% <0.00%> (ø)
src/goto-programs/validate_goto_model.cpp	100.00% <ø> (ø)
src/goto-symex/frame.h	100.00% <ø> (ø)
src/goto-symex/goto_symex.h	92.30% <ø> (ø)
unit/goto-programs/goto_program_validate.cpp	100.00% <ø> (ø)
src/crangler/c_defines.cpp	41.66% <41.66%> (ø)
src/crangler/crangler_parse_options.cpp	56.25% <56.25%> (ø)
src/crangler/c_wrangler.cpp	58.29% <58.29%> (ø)
src/crangler/scanner.l	63.63% <63.63%> (ø)
... and 23 more

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