[analyzer] IDC: Add config option; perform the idc check on first “null node” rather than last “non-null”.

The second modification does not lead to any visible result, but, theoretically, is what we should
have been looking at to begin with since we are checking if the node was assumed to be null in
an inlined function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176576 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Pass the correct Expr to the bug reporter visitors when dealing with CompoundLiteralExpr

This allows us to trigger the IDC visitor in the added test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176577 91177308-0d34-0410-b5e6-96231b3b80d8