libstdc++: Simplify _Hashtable merge functions

I realised that _M_merge_unique and _M_merge_multi call extract(iter)
which then has to call _M_get_previous_node to iterate through the
bucket to find the node before the one iter points to. Since the merge
function is already iterating over the entire container, we had the
previous node a moment ago. Walking the whole bucket to find it again is
wasteful. We could just rewrite the loop in terms of node pointers
instead of iterators, and then call _M_extract_node directly. However,
this is only possible when the source container is the same type as the
destination, because otherwise we can't access the source's private
members (_M_before_begin, _M_begin, _M_extract_node etc.)

Add overloads of _M_merge_unique and _M_merge_multi that work with
source containers of the same type, to enable this optimization.

For both overloads of _M_merge_unique we can also remove the conditional
modifications to __n_elt and just consistently decrement it for every
element processed. Use a multiplier of one or zero that dictates whether
__n_elt is passed to _M_insert_unique_node or not. We can also remove
the repeated calls to size() and just keep track of the size in a local
variable.

Although _M_merge_unique and _M_merge_multi should be safe for
"self-merge", i.e. when doing c.merge(c), it's wasteful to search/insert
every element when we don't need to do anything. Add 'this == &source'
checks to the overloads taking an lvalue of the container's own type.
Because those checks aren't needed for the rvalue overloads, change
those to call the underlying _M_merge_xxx function directly instead of
going through the lvalue overload that checks the address.

I've also added more extensive tests for better coverage of the new
overloads added in this commit.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_M_merge_unique): Add overload for
	merging from same type.
	(_M_merge_unique<Compatible>): Simplify size tracking. Add
	comment.
	(_M_merge_multi): Add overload for merging from same type.
	(_M_merge_multi<Compatible>): Add comment.
	* include/bits/unordered_map.h (unordered_map::merge): Check for
	self-merge in the lvalue overload. Call _M_merge_unique directly
	for the rvalue overload.
	(unordered_multimap::merge): Likewise.
	* include/bits/unordered_set.h (unordered_set::merge): Likewise.
	(unordered_multiset::merge): Likewise.
	* testsuite/23_containers/unordered_map/modifiers/merge.cc:
	Add more tests.
	* testsuite/23_containers/unordered_multimap/modifiers/merge.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/modifiers/merge.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/modifiers/merge.cc:
	Likewise.