[Sema][ObjC] Don't warn about an implicitly retained self if the

retaining block and all of the enclosing blocks are non-escaping.

If the block implicitly retaining self doesn't escape, there is no risk
of creating retain cycles, so clang shouldn't diagnose it and force
users to add self-> to silence the diagnostic.

Also, fix a bug where clang was failing to diagnose an implicitly
retained self inside a c++ lambda nested inside a block.

rdar://problem/25059955

Differential Revision: https://reviews.llvm.org/D60736

llvm-svn: 358624