C++ front-end: support explicit type conversion with braced-init-list

[tautschnig]

Now follows the grammar described in the C++ standard: Explicit type conversions are postfix expressions. This required adding rules for C++ cast expressions, which in turn simplifies type checking for we no longer treat them as function calls with template arguments.

• 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).
• 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 83.33333% with 21 lines in your changes missing coverage. Please review.

Project coverage is 78.36%. Comparing base (82bbeb8) to head (fe1c657).

Files	Patch %	Lines
src/cpp/parse.cpp	83.63%	18 Missing ⚠️
src/ansi-c/scanner.l	60.00%	2 Missing ⚠️
src/cpp/cpp_typecheck_expr.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8385      +/-   ##
===========================================
+ Coverage    78.32%   78.36%   +0.03%     
===========================================
  Files         1726     1726              
  Lines       188506   188464      -42     
  Branches     18249    18319      +70     
===========================================
+ Hits        147646   147685      +39     
+ Misses       40860    40779      -81     

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