Revert r70876 and add a testcase (@c7) showing the problem:

Revert r70876 and add a testcase (@c7) showing the problem:
bits captured, but the pointer marked nocapture.  In fact
I now recall that this problem is why only readnone functions
returning void were considered before!  However keep a small
fix that was also in r70876: a readnone function returning
void can result in bits being captured if it unwinds, so
test for this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71168 91177308-0d34-0410-b5e6-96231b3b80d8