libstdc++: Make std::rethrow_if_nested work without RTTI

This allows std::rethrow_if_nested to work with -fno-rtti by not
attempting the dynamic_cast if it requires RTTI, since that's ill-formed
with -fno-rtti. The cast will still work if a static upcast to
std::nested_exception is allowed.

Also use if-constexpr to avoid the compile-time overload resolution (and
SFINAE) and run-time dispatching for std::rethrow_if_nested and
std::throw_with_nested.

Also add better doxygen comments throughout the file.

libstdc++-v3/ChangeLog:

	* libsupc++/nested_exception.h (throw_with_nested) [C++17]: Use
	if-constexpr instead of tag dispatching.
	(rethrow_if_nested) [C++17]: Likewise.
	(rethrow_if_nested) [!__cpp_rtti]: Do not use dynamic_cast if it
	would require RTTI.
	* testsuite/18_support/nested_exception/rethrow_if_nested-term.cc:
	New test.

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