libstdc++: Check for overflow in regex back-reference [PR106607]

libstdc++: Check for overflow in regex back-reference [PR106607]

Currently we fail to notice integer overflow when parsing a
back-reference expression, or when converting the parsed result from
long to int. This changes the result to be int, so no conversion is
needed, and uses the overflow-checking built-ins to detect an
out-of-range back-reference.

libstdc++-v3/ChangeLog:

	PR libstdc++/106607
	* include/bits/regex_compiler.tcc (_Compiler::_M_cur_int_value):
	Use built-ins to check for integer overflow in back-reference
	number.
	* testsuite/28_regex/basic_regex/106607.cc: New test.