Add some documentation stating that the memory allocated by the ASTContext.h placement new does not need to be explicitly freed

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5392

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218232 91177308-0d34-0410-b5e6-96231b3b80d8