[analyzer] Suppress stack address escape on CK_CopyAndAutoreleaseBlockObject.

Don't warn about addresses of stack-allocated blocks escaping if the block
region was cast with CK_CopyAndAutoreleaseBlockObject. These casts, which
are introduced in the implicit conversion operator for lambda-to-block
conversions, cause the block to be copied to the heap -- so the warning is
spurious.

llvm-svn: 254639