[CodeGen][ObjC] Make block copy/dispose helper functions exception-safe.

When an exception is thrown in a block copy helper function, captured
objects that have previously been copied should be destructed or
released. Similarly, captured objects that are yet to be released should
be released when an exception is thrown in a dispose helper function.

rdar://problem/42410255

Differential Revision: https://reviews.llvm.org/D49718

llvm-svn: 338041