-Wuninitialized should not warn about variables captured by blocks as byref.

-Wuninitialized should not warn about variables captured by blocks as byref.

Note this can potentially be enhanced to detect if the __block variable
is actually written by the block, or only when the block "escapes" or
is actually used, but that requires more analysis than it is probably worth
for this simple check.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@128681 91177308-0d34-0410-b5e6-96231b3b80d8