Add an option to disable tail-call optimization for escaping blocks.

This makes it easier to debug crashes and hangs in block functions since
users can easily find out where the block is called from. The option
doesn't disable tail-calls from non-escaping blocks since non-escaping
blocks are not as hard to debug as escaping blocks.

rdar://problem/35758207

Differential Revision: https://reviews.llvm.org/D43841

llvm-svn: 326530