C/C++ front-end: void-typed symbols are permitted when extern

[tautschnig]

This leaves it to the linker to resolve to some other type, and is
quietly accepted by GCC. Required for building the Linux kernel. The
changes in 9018306 were overly strict.

• 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 #5732 (f0dd97d) into develop (36fa6f4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5732   +/-   ##
========================================
  Coverage    69.51%   69.51%           
========================================
  Files         1243     1243           
  Lines       100701   100701           
========================================
  Hits         70000    70000           
  Misses       30701    30701           

Flag	Coverage Δ
cproversmt2	43.28% <33.33%> (ø)
regression	66.40% <100.00%> (ø)
unit	32.25% <33.33%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.cpp	81.65% <100.00%> (ø)
src/cpp/cpp_typecheck_code.cpp	72.47% <100.00%> (ø)
src/cpp/cpp_typecheck_declaration.cpp	73.84% <100.00%> (ø)

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