libstdc++: Simplify std::shared_ptr construction from std::weak_ptr

The _M_add_ref_lock() and _M_add_ref_lock_nothrow() members of
_Sp_counted_base are very similar, except that the former throws an
exception when the use count is zero and the latter returns false. The
former (and its callers) can be implemented in terms of the latter.
This results in a small reduction in code size, because throwing an
exception now only happens in one place.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr.h (shared_ptr(const weak_ptr&, nothrow_t)):
	Add noexcept.
	* include/bits/shared_ptr_base.h (_Sp_counted_base::_M_add_ref_lock):
	Remove specializations and just call _M_add_ref_lock_nothrow.
	(__shared_count, __shared_ptr): Use nullptr for null pointer
	constants.
	(__shared_count(const __weak_count&)): Use _M_add_ref_lock_nothrow
	instead of _M_add_ref_lock.
	(__shared_count(const __weak_count&, nothrow_t)): Add noexcept.
	(__shared_ptr::operator bool()): Add noexcept.
	(__shared_ptr(const __weak_ptr&, nothrow_t)): Add noexcept.

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