Allocate stack storage for .block_descriptor and captured self at -O0.
This way the register allocator will not optimize away the debug info
for captured variables.

Fixes rdar://problem/12767564

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177086 91177308-0d34-0410-b5e6-96231b3b80d8