Remove addBlockByrefAddress(), it is dead code as far as clang is concerned.

This patch removes addBlockByrefAddress(), it is dead code as far as
clang is concerned: Every byref block capture is emitted with a
complex expression that is equivalent to what this function does.

rdar://problem/31629055

Differential Revision: https://reviews.llvm.org/D51763

llvm-svn: 341737