libstdc++: Add macros for the inline namespace std::_V2

Use macros to open and close the inline namespace _V2 that is used for
ABI versioning of individual components such as chrono::system_clock.

This allows the namespace to be hidden in the docs generated by Doxygen,
so that we document std::foo instead of std::_V2::foo.

This also makes it easy to remove that namespace entirely for the
gnu-versioned-namespace build, where everything is already versioned as
std::__8 and there are no backwards compatibility guarantees.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Expand new macros to
	nothing.
	* include/bits/c++config (_GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE)
	(_GLIBCXX_END_INLINE_ABI_NAMESPACE): Define new macros.
	* include/bits/algorithmfwd.h (_V2::__rotate): Use new macros
	for the namespace.
	* include/bits/chrono.h (chrono::_V2::system_clock): Likewise.
	* include/bits/stl_algo.h (_V2::__rotate): Likewise.
	* include/std/condition_variable (_V2::condition_variable_any):
	Likewise.
	* include/std/system_error (_V2::error_category): Likewise.

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