Fix memory leak for APValues that do memory allocation.

This patch ensures that APValues are deallocated with the ASTContext by
registering a deallocation function for APValues to the ASTContext.

Original version of the patch by James Dennett.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@183101 91177308-0d34-0410-b5e6-96231b3b80d8