Call destroy from ~BasicCallGraph.

This fix a memory leak found by valgrind.

Calling it from the base class destructor would not destroy the BasicCallGraph
bits.

FIXME: BasicCallGraph is the only thing that inherits from CallGraph. Can
we merge the two?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193412 91177308-0d34-0410-b5e6-96231b3b80d8