libstdc++: Add attributes to <system_error> and related

Add the const attribute to std::future_category() and
std::iostream_category(), to match the existing attributes on
std::generic_category() and std::system_category().

Also add [[nodiscard]] to those functions and to the comparison
operators for std::error_code and std::error_condition, and to
std::make_error_code and std::make_error_condition overloads.

libstdc++-v3/ChangeLog:

	* include/bits/ios_base.h (io_category): Add const and nodiscard
	attributes.
	(make_error_code, make_error_condition): Add nodiscard.
	* include/std/future (future_category): Add const and nodiscard.
	(make_error_code, make_error_condition): Add nodiscard.
	* include/std/system_error (generic_category system_category):
	Add nodiscard. Replace _GLIBCXX_CONST with C++11 attribute.
	(error_code::value, error_code::category, error_code::operator bool)
	(error_condition::value, error_condition::category)
	(error_condition::operator bool, make_error_code)
	(make_error_condition, operator==, operator!=, operator<=>): Add
	nodiscard.

0 files changed, 0 insertions, 0 deletions