Add a destruct-on-exit function to ASTContext.

Add a destruct-on-exit function to ASTContext.

It looks like the only use of AddDeallocation is to indirectly call the
destructors of objects. In one case I found
(TypeAliasTemplateDecl::Common), the destructor is a nop, so registering
it to run later seems pointless.

All of the other *::Common types have non-trivial dtors, so deleting the
useless AddDeallocation felt somewhat fragile. Happy to kill it + turn
the is_trivial_dtor check into a static_assert if people think that'd be
better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295029 91177308-0d34-0410-b5e6-96231b3b80d8