Objective-C ARC. Blocks that strongly capture themselves
to call themselves will get the warning:
"Capturing <itself> strongly in this block is likely to
lead to a retain cycle". Cut down on the amount of noise
by noticing that user at some point sets the captured variable
to null in order to release it (and break the cycle). 
// rdar://16944538


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210823 91177308-0d34-0410-b5e6-96231b3b80d8
Tweak documentation.

1. Having "get started", "get involved", and "hacking" makes it hard to find
   how to send patches, so add a link from "get involved" to "hacking".
2. Remove an almost 5 year old note on the test running meachanism changing
   soon.
3. Let "hacking" link to the LLVM developer policy.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210826 91177308-0d34-0410-b5e6-96231b3b80d8
Prefix another use of error_code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210832 91177308-0d34-0410-b5e6-96231b3b80d8
s/pr2007/20007/ in a test

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210834 91177308-0d34-0410-b5e6-96231b3b80d8