C++ front-end: support constexpr

[tautschnig]

Mark constexpr symbols as macros and use direct replacement (non-function symbols) or in-place evaluation (uses of constexpr function symbols).

• 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

Attention: Patch coverage is 79.45205% with 15 lines in your changes missing coverage. Please review.

Project coverage is 77.91%. Comparing base (82bbeb8) to head (514db34).

Files	Patch %	Lines
src/cpp/cpp_typecheck_expr.cpp	84.21%	6 Missing ⚠️
src/cpp/parse.cpp	50.00%	6 Missing ⚠️
src/cpp/cpp_storage_spec.cpp	50.00%	1 Missing ⚠️
src/cpp/cpp_storage_spec.h	87.50%	1 Missing ⚠️
src/cpp/cpp_typecheck_resolve.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8386      +/-   ##
===========================================
- Coverage    78.32%   77.91%   -0.41%     
===========================================
  Files         1726     1726              
  Lines       188506   189560    +1054     
  Branches     18249    18255       +6     
===========================================
+ Hits        147646   147704      +58     
- Misses       40860    41856     +996     

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