Have the constant manager take ownership of constants. (#1866)

* Have the constant manager take ownership of constants.

Right now the owner of an object of type contant that is in the
|const_pool_| of the constant manager is unclear.  The constant
manager does not delete them, there is no other reasonable owner.  This
causes memory leaks.

This change fixes the memory leaks by having the constant manager
take ownership of the constant that is stores in |const_pool_|.  Other
changes include interface changes to make it explicit that the constant
manager takes ownership of the object when a constant is registered
with the constant manager.

Fixes #1865.