[analyzer] Flush bug reports in deterministic order.

This makes the diagnostic output order deterministic. 
1) This makes order of text diagnostics consistent from run to run.

2) Also resulted in different bugs being reported (from one run to
another) with plist-html output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161151 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed