[AST] Forbid copy/move of statements/types

Statements, expressions and types are not supposed to be copied/moved,
and trying to do so is only going to result in tears. Someone tripped
on this a few days ago on the mailing list. NFC.

Differential Revision: https://reviews.llvm.org/D60123

Reviewed By: aaron.ballman

llvm-svn: 358283