libstdc++: Add inline to maybe-constexpr functions (PR 92927)

libstdc++: Add inline to maybe-constexpr functions (PR 92927)

Originally these functions were always inline. I changed them in r277342
to be always constexpr, then in r277588 changed them to be constexpr for
C++14, but I didn't restore the 'inline' for C++11. That leads to linker
errors when libstdc++.so is built unoptimized, because those functions
don't get instantiated in src/c++11/string-inst.o

	PR libstdc++/92927
	* include/bits/alloc_traits.h (__alloc_on_copy, __alloc_on_move)
	(__alloc_on_swap): Add inline specifier.

From-SVN: r279656