Suppress GCC -Weffc++ and -Wctor-dtor-privacy warnings in public headers

[SNelcha]

This pull request addresses warning leakage from Catch2 internals when users include Catch2 headers under strict compiler warning flags.
The change adds GCC-specific suppression macros for -Weffc++ and -Wctor-dtor-privacy in src/catch2/internal/catch_compiler_capabilities.hpp, and applies them at major public header boundaries so these warnings are contained within Catch2 rather than surfacing in downstream user builds.
The updated public entry headers are:
src/catch2/catch_test_macros.hpp
src/catch2/catch_template_test_macros.hpp
src/catch2/catch_session.hpp
src/catch2/catch_all.hpp
src/catch2/matchers/catch_matchers.hpp
src/catch2/generators/catch_generators.hpp
This is a minimal, boundary-based fix intended to align with Catch2’s existing warning suppression infrastructure rather than refactoring all internal warning-triggering patterns.
Validation performed:
cmake --build build succeeded
ctest --test-dir build --output-on-failure passed (75/75)
compile probes across the major public headers passed with -Weffc++ -Wctor-dtor-privacy -Werror in the local environment

[horenmar]

If you have specific changes to propose I will look over them, but the Weffc++ implementation quality is too low to bother supporting it. See warnings like these:

class ScopedElement {
public:
    ScopedElement( XmlWriter* writer CATCH_ATTR_LIFETIMEBOUND, XmlFormatting fmt );

    ScopedElement( ScopedElement&& other ) noexcept;
    ScopedElement& operator=( ScopedElement&& other ) noexcept;
    ~ScopedElements();
    // ...
}; 

/mnt/c/ubuntu/Catch2/src/catch2/../catch2/internal/catch_xmlwriter.hpp: At global scope:
/mnt/c/ubuntu/Catch2/src/catch2/../catch2/internal/catch_xmlwriter.hpp:63:15: warning: ‘class Catch::XmlWriter::ScopedElement’ has pointer data members [-Weffc++]
   63 |         class ScopedElement {
      |               ^~~~~~~~~~~~~
/mnt/c/ubuntu/Catch2/src/catch2/../catch2/internal/catch_xmlwriter.hpp:63:15: warning:   but does not declare ‘Catch::XmlWriter::ScopedElement(const Catch::XmlWriter::ScopedElement&)’ [-Weffc++]
/mnt/c/ubuntu/Catch2/src/catch2/../catch2/internal/catch_xmlwriter.hpp:63:15: warning:   or ‘operator=(const Catch::XmlWriter::ScopedElement&)’ [-Weffc++]
/mnt/c/ubuntu/Catch2/src/catch2/../catch2/internal/catch_xmlwriter.hpp:93:24: note: pointer member ‘Catch::XmlWriter::ScopedElement::m_writer’ declared here
   93 |             XmlWriter* m_writer = nullptr;
      |                        ^~~~~~~~        

Oh the class doesn't declare copy operators? Duh, it has move operators. Similarly, the warning doesn't handle classes deriving from NonCopyable to disable copy/move ops.

---

Another example:

class MessageHolder {
    // The actual message vector passed to the reporters
    std::vector<MessageInfo> messages;
    // IDs of messages from UNSCOPED_X macros, which we have to
    // remove manually.
    std::vector<unsigned int> unscoped_ids;

    // ...
};

/mnt/c/ubuntu/Catch2/src/catch2/internal/catch_run_context.cpp: In constructor ‘Catch::Detail::MessageHolder::MessageHolder()’:
/mnt/c/ubuntu/Catch2/src/catch2/internal/catch_run_context.cpp:240:15: warning: ‘Catch::Detail::MessageHolder::messages’ should be initialized in the member initialization list [-Weffc++]
  240 |         class MessageHolder {
      |               ^~~~~~~~~~~~~
/mnt/c/ubuntu/Catch2/src/catch2/internal/catch_run_context.cpp:240:15: warning: ‘Catch::Detail::MessageHolder::unscoped_ids’ should be initialized in the member initialization list [-Weffc++]

No, I am not going to explicitly initialize std::vector, when it has perfectly good default initialization.