libstdc++: Add noexcept-specifier to std::reference_wrapper::operator()

This isn't required by the standard, but there's an LWG issue suggesting
to add it.

Also use __invoke_result instead of result_of, to match the spec in
recent standards.

libstdc++-v3/ChangeLog:

	* include/bits/refwrap.h (reference_wrapper::operator()): Add
	noexcept-specifier and use __invoke_result instead of result_of.
	* testsuite/20_util/reference_wrapper/invoke-noexcept.cc: New test.