[CodeGen] Look at the type of a block capture field rather than the type

of the captured variable when determining whether the capture needs
special handing when the block is copied or disposed.

This fixes bugs in the handling of variables captured by a block that is
nested inside a lambda that captures the variables by reference.

rdar://problem/43540889

Differential Revision: https://reviews.llvm.org/D51025

llvm-svn: 340408