libstdc++: contracts support changes

libstdc++-v3/ChangeLog:

	* src/c++17/contract.cc: Remove member functions.
	* config/abi/pre/gnu.ver: Remove their symbols, move to latest
	version.
	* include/std/contract: Make them inline.
	Change string_view data members to const char *.
	Use libstdc++ naming pattern.
3 files changed