Avoid needlessly copying a block to the heap when a block literal

Avoid needlessly copying a block to the heap when a block literal
initializes a local auto variable or is assigned to a local auto
variable that is declared in the scope that introduced the block
literal.

rdar://problem/13289333

https://reviews.llvm.org/D58514

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@355012 91177308-0d34-0410-b5e6-96231b3b80d8