clang-tidy.yml: updated to Clang 21

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/usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/stl_pair.h:295:17: error: an exception may be thrown in function 'pair' which should not throw exceptions [bugprone-exception-escape,-warnings-as-errors]
  295 |       constexpr pair(pair&&) = default;         ///< Move constructor
      |                 ^
/home/runner/work/simplecpp/simplecpp/simplecpp.cpp:1754:29: note: frame #0: unhandled exception of type 'simplecpp::Macro::Error' may be thrown in function 'parseDefine' here
 1754 |                             throw Error(tok->location, "In definition of '" + nameTokDef->str() + "': Missing opening parenthesis for __VA_OPT__");
      |                             ^
/home/runner/work/simplecpp/simplecpp/simplecpp.cpp:1515:21: note: frame #1: function 'operator=' calls function 'parseDefine' here
 1515 |                     parseDefine(other.nameTokDef);
      |                     ^
/home/runner/work/simplecpp/simplecpp/simplecpp.cpp:1502:19: note: frame #2: function 'Macro' calls function 'operator=' here
 1502 |             *this = other;
      |                   ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/stl_pair.h:295:17: note: frame #3: function 'pair' calls function 'Macro' here
  295 |       constexpr pair(pair&&) = default;         ///< Move constructor
      |                 ^

@glankk this was introduced in 8f8bfe3 - please have a look

[glankk]

@glankk this was introduced in 8f8bfe3 - please have a look

The root cause of this issue seems to be that Macro doesn't have a move constructor, so its copy constructor is used instead in the std::pair move constructor. The default move constructor in Macro is deleted because it has a reference as a member (which I think is generally a bad idea).

I think the cleanest solution to this is to replace the references with pointers and implement sane move operators. I'll work out a draft. @danmar please chime in if you have an opinion on this.

[glankk]

Related: #493

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I filed https://trac.cppcheck.net/ticket/14119 about detecting one of the misc-const-correctness cases ourselves.

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I worked around the bugprone-exception-escape warning for now and filed #537.

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I filed llvm/llvm-project#157730 about this.