Introduce needsCleanup() for APFloat and APInt.

This is needed in clang so one can check if the object needs the
destructor called after its memory was freed. This is useful when
creating many APInt/APFloat objects with placement new, where the
overhead of tracking the pointers for cleanup is significant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183100 91177308-0d34-0410-b5e6-96231b3b80d8