libstdc++: Fix misuse of alloca in std::bitset [PR108214]

libstdc++: Fix misuse of alloca in std::bitset [PR108214]

The use of alloca in a constructor is wrong, because the memory is gone
after the constructor returns, and will be overwritten by a subsequent
function call. This didn't show up in testing because function inlining
alters the stack usage.

libstdc++-v3/ChangeLog:

	PR libstdc++/108214
	* include/std/bitset (operator>>): Use alloca in the right
	scope, not in a constructor.
	* testsuite/20_util/bitset/io/input.cc: Check case from PR.