C front-end: __builtin_shuffle, __builtin_shufflevector

[tautschnig]

These are polymorphic built-ins supported by GCC and Clang,
respectively. Introduce a new shuffle_vector expression to handle these,
with type checking addressing the differences between the GCC/Clang
APIs.

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
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• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5913 (de4557f) into develop (95e1161) will decrease coverage by 0.00%.
The diff coverage is 64.80%.

@@             Coverage Diff             @@
##           develop    #5913      +/-   ##
===========================================
- Coverage    75.02%   75.01%   -0.01%     
===========================================
  Files         1431     1433       +2     
  Lines       155595   155701     +106     
===========================================
+ Hits        116734   116806      +72     
- Misses       38861    38895      +34     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
...rc/ansi-c/c_typecheck_gcc_polymorphic_builtins.cpp	76.58% <45.07%> (-4.36%)	⬇️
src/ansi-c/c_typecheck_expr.cpp	75.23% <66.66%> (-0.06%)	⬇️
src/ansi-c/c_expr.cpp	96.55% <96.55%> (ø)
src/ansi-c/c_expr.h	100.00% <100.00%> (ø)
src/solvers/smt2/smt2_parser.cpp	70.87% <0.00%> (+0.06%)	⬆️
...olvers/strings/string_constraint_instantiation.cpp	94.69% <0.00%> (+3.53%)	⬆️

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[kroening]

I would put the new expressions into ansi-c/c_expr.h and .cpp -- while it's conceivable that there might be another frontend with expressions like these, it's specific dialects of C for now.