Fix splay tree KEY leak detected in GDB test gdb.base/macscp.exp

Fix splay tree KEY leak detected in GDB test gdb.base/macscp.exp

When a node is removed from a splay tree, the splay tree was
not using the function splay_tree_delete_key_fn to release the key.
This was causing a leak, fixed by Tom Tromey.

This patch fixes another key leak, that happens when a key equal to
a key already present is inserted.  In such a case, we have to release
the old KEY.
Note that this is based on the assumption that the caller always
allocates a new KEY when doing an insert.

Also, clarify the documentation about when the release functions are
called.

2019-02-11  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* splay-tree.h (splay_tree_delete_key_fn): Update comment.
	(splay_tree_delete_value_fn): Likewise.

libiberty/ChangeLog
2019-02-11  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* splay-tree.c (splay_tree_insert): Also release old KEY in case
	of insertion of a key equal to an already present key.
	(splay_tree_new_typed_alloc): Update comment.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@268793 138bc75d-0d04-0410-961f-82ee72b054a4