libstdc++: Add noexcept to functions in <regex>

libstdc++: Add noexcept to functions in <regex>

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/regex.h (basic_regex, swap): Add noexcept to
	non-throwing functions.
	* include/bits/regex_automaton.h (_State_base, _State)
	(_NFA_base): Likewise.
	* include/bits/regex_compiler.h (_Compiler): Likewise.
	* include/bits/regex_error.h (regex_error::code()): Likewise.
	* include/bits/regex_scanner.h (_Scanner): Likewise.

(cherry picked from commit df0dd04b78cfc0f723387b703978600caac93cbb)