Devirtualize Constant::destroyConstant.

This reorganizes destroyConstant and destroyConstantImpl.

Now there is only destroyConstant in Constant itself, while
subclasses are required to implement destroyConstantImpl.

destroyConstantImpl no longer calls delete but is instead only
responsible for removing the constant from any maps in which it
is contained.

Reviewed by Duncan Exon Smith.

llvm-svn: 240471