Guard cxxabi.h include with __has_include for clang-cl (#6145)

* fix: guard cxxabi.h include with __has_include for clang-cl

clang-cl on Windows defines __GNUG__ but does not ship <cxxabi.h>,
making detail/typeid.h fail to compile. Introduce PYBIND11_HAS_CXXABI_H,
defined only when <cxxabi.h> is actually available (__has_include with
a fallback for compilers without it, including C++11), and use it to
guard both the include and the abi::__cxa_demangle call site.

Fixes #6129

* docs: explain the __has_include fallback for GCC < 5

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Co-authored-by: aidaodedjl <jialiang.dong@ju-shen.com>