libstdc++: Fix some warnings in headers

libstdc++: Fix some warnings in headers

These are usually suppressed without -Wsystem-headers.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_Local_iterator_base): Cast
	value to avoid -Wsign-compare warnings.
	* include/bits/regex.h (sub_match::_M_str): Avoid narrowing
	conversion.
	* include/bits/regex_compiler.tcc (_Compiler::_M_quantifier):
	Initialize variable to avoid -Wmaybe-uninitialized warning.
	* include/bits/shared_ptr_base.h (_Sp_counted_deleter::_Impl):
	Reorder mem-initializer-list to avoid -Wreorder warning.
	* include/bits/stl_tree.h (_Rb_tree_impl): Explicitly
	initialize base class in copy constructor.
	* include/debug/safe_iterator.h (_Safe_iterator): Likewise.
	* include/ext/debug_allocator.h: Reorder mem-initializer-list
	to avoid -Wreorder warning.
	* include/ext/throw_allocator.h (throw_allocator_limit)
	(throw_allocator_random): Add user-declared assignment operators
	to avoid -Wdeprecated-copy warnings.