Fix overflows in std::pmr::unsynchonized_pool_resource

Fix overflows in std::pmr::unsynchonized_pool_resource

	* src/c++17/memory_resource.cc (bitset::full()): Handle edge case
	for _M_next_word maximum value.
	(bitset::get_first_unset(), bitset::set(size_type)): Use
	update_next_word() to update _M_next_word.
	(bitset::update_next_word()): New function, avoiding wraparound of
	unsigned _M_next_word member.
	(bitset::max_word_index()): New function.
	(chunk::chunk(void*, uint32_t, void*, size_t)): Add assertion.
	(chunk::max_bytes_per_chunk()): New function.
	(pool::replenish(memory_resource*, const pool_options&)): Prevent
	_M_blocks_per_chunk from exceeding max_blocks_per_chunk or from
	causing chunk::max_bytes_per_chunk() to be exceeded.
	* testsuite/20_util/unsynchronized_pool_resource/allocate-max-chunks.cc:
	New test.

From-SVN: r266087