[CodeGen] Avoid ctor/dtor boilerplate with some C++11

Non-owning pointers that cache LLVM types and constants can use
'nullptr' default member initializers so that we don't need to mention
them in the constructor initializer list.

Owning pointers should use std::unique_ptr so that we don't need to
manually delete them in the destructor. They also don't need to be
mentioned in the constructor at that point.

NFC

llvm-svn: 266263